Firing a constant barrage of artillery at the Russians, a troop of Ukrainian soldiers are keeping up morale: their shelling has so far stopped Moscow's tanks entering Kyiv from the east. 

And they hope that the enemy forces, which have struggled to take large cities, will eventually get bogged down.

Wearing camouflage uniform and carrying a Kalashnikov, a soldier nicknamed Topaz watches the motorway running north from Kyiv's eastern suburbs. 

With his companions, he is keeping up a barrage of artillery from behind the front line, as regular explosions of heavy weaponry echo across the plain and an icy wind blows.

"Today the troops' morale is at its highest," said the muscular man in his 40s, radioing messages via a headset.

"The day before yesterday, we pushed back the Russian tanks" by a few hundred metres, he says, smiling, in a pause between two exchanges of fire.

A video of the feat, captured by a Ukrainian military drone and posted on social media, shows a column of about 20 Russian vehicles, many of them tanks, moving southwards along the motorway.

A sudden hail of shells and rockets hits the Russian convoy, blowing up first the lead tank and then one at the rear.

"We shot at them with tanks and rocket launchers," Topaz says.

The Russian convoy makes a sudden half-turn and gets out of the village as quickly as it can.

"They went to hide in the nearby villages," says Topaz.

The attack took place close to the village of Skybyn in the Kyiv region, a few kilometres from the road into Brovary, the eastern gateway into the capital.

- Cities harder to win -

According to several Ukrainian soldiers, at least two Russian vehicles, one of them a tank, were destroyed -- which the video appears to support -- and a Russian tank commander was killed.

On the Ukrainian side, "we had five wounded, one of whom is in a critical condition", after the Russian side opened fire in response, says one soldier, Ilya Berezenko, 27.

Since the invasion began on March 24, the Russian troops have seized numerous pockets of strategically important territory, particularly along the southern coast and to the east and north and are now positioned at Kyiv's northwestern and eastern edges.

But they seem to have run into difficulties in winning large cities such as Chernigiv or Kharkiv and in the northeastern Kyiv suburbs of Bucha and Irpin, as well as in the east.

While Russia's bombardment is massive and deadly, raising fears it will lay waste to cities as it did in Chechnya and Syria, the ground troops around Kyiv particularly seem to have hardly moved in recent days, according to a number of military experts. 

To the east of Kyiv, Russian tanks a few days ago reached a point a few kilometres from Brovary, with the initial impression being that they had driven 150 kilometres (90 miles) from Belarus in the north, a close ally they can use as a rear base.

- 'They'll wear out' -

But Ukrainian military and numerous defence experts say the Russians actually travelled from the east and from Sumy some 300 kilometres away, near the Russian border, taking small roads through sparsely populated areas. 

At the start of the week, the Russian tanks arrived a few kilometres from Brovary and positioned themselves in surrounding villages. Locals said they counted more than 70 military vehicles, including around 40 tanks and around 300 soldiers.

This column, or part of it, then moved towards Brovary and Kyiv and was halted Thursday at Skybyn, according to Ukrainian military experts, who question whether the Russians seriously thought they could use a few dozen tanks to enter Kyiv -- a city of thre million prior to the invasion and now fortified with barricades.

"I don't know why they are doing that," says Berezenko.

"You'd have to be mad, or misinformed... Or else they just want to confuse us." 

The incident in Skybyn reinforces Ukrainian soldiers' belief that the Russians have overestimated their resources, in terms of troops and equipment, and underestimated those of their opponent.

"They have to camp in villages in temperatures of nearly minus 10 Celsius at night. They lack provisions and have to raid houses," adds the Ukrainian soldier.

The Russians nevertheless have advanced far enough to raise fears of Kyiv becoming encircled imminently. In the capital, only the roads to the south are still open and the city is preparing to mount a "relentless defence", according to the Ukrainian presidency.

The Russians could continue to destroy and bomb Ukraine, but they "don't have enough men or equipment to occupy the country," says Berezenko, giving his view that Moscow's soldiers are "not numerous enough and (are) far from home" and "will end up wearing themselves out".

Alarm grows over Mariupol as Russia squeezes Kyiv

Russian forces upped the pressure on Kyiv Saturday and pummelled civilian areas in other Ukrainian cities, amid fresh efforts to deliver aid to the devastated port city of Mariupol.

Both Ukrainian and Russian officials on Saturday described the quickly worsening humanitarian situation as "catastrophic."

In Moscow, the defence ministry described an unrelenting push on the ground, reporting that Russian forces had advanced 12 kilometers (seven miles) over "a broad front" during the day, without specifying exactly where. 

It said pro-Russian separatists in the east had reached the edge of Severodonetsk, a city of 100,000.

Russian strikes have destroyed the airport in the town of Vasylkiv, south of Kyiv, the mayor said.

The northwest suburbs of the capital, including Irpin and Bucha, have endured days of heavy bombardment while Russian armoured vehicles are advancing on the city's northeastern edge.

Kyiv Mayor Vitali Klitschko said the capital, described by a senior Ukrainian official Friday as a "city under siege", was reinforcing defences and stockpiling food and medicine.

Buses were continuing to bring refugees into the city from the hard-hit suburbs, Klitschko said in a video message, adding: "We will not give up." 

Other cities have already fallen or been surrounded since Russia invaded its neighbour on February 24, with civilians targeted in what the United Nations warned could amount to war crimes.

The southern port city Mariupol in particular is facing what Ukraine says is a "humanitarian catastrophe", with more than 1,500 civilians killed over 12 days.

A top Russian officer described the situation in similarly stark language.

"Unfortunately, the humanitarian situation in Ukraine is continuing to deteriorate rapidly, and in some cities it has reached catastrophic proportions," said the head of the Russian National Defence Control Centre, Mikhail Mizintsev.

But some Ukrainian soldiers remained hopeful: one in Kyiv, 27-year-old Ilya Berezenko, said he doesn't believe Moscow has "enough men or equipment to occupy" Ukraine.

Russia's soldiers are "not numerous enough and (are) far from home" he told AFP, adding that they "will end up wearing themselves out".

As intense diplomatic efforts continued, the leaders of France and Germany, Emmanuel Macron and Olaf Scholz, urged Russian counterpart Vladimir Putin during a phone conversation to end the deadly blockade, Paris said.

Prompting a small glimmer of hope, Ukraine President Volodymyr Zelensky said Saturday that Russia -- after appearing unbudging for days -- had adopted a "fundamentally different approach" in the latest talks to end the conflict. 

He told reporters he was "happy to have a signal from Russia" after Putin spoke of "some positive shifts" in a near-daily dialogue.

Elsewhere across the increasingly desperate country, there were only scattered signs of progress.

A humanitarian convoy loaded with 90 tonnes of food and medicine left the town of Zaporizhzhia for Mariupol on Saturday, according to local officials, with hopes that it will be able to evacuate civilians on the way back.

Orthodox clergy volunteered to accompany the convoy, they said, after Zelensky accused Russia of targeting previous similar efforts.

Some evacuation efforts have been fruitful. According to Deputy Prime Minister Iryna Vereshchuk, nine of the 14 humanitarian corridors opened were successful. 

Nearly 13,000 people were able to evacuate on Saturday on those routes, while the remaining five were blocked by factors such as Russian troops or shooting, she said.

Russian troops shot at a group of women and children leaving a village near Kyiv, killing seven, including a child, the Ukrainian military intelligence service reported.

- Foreign pressure grows -

The United Nations estimates that almost 2.6 million people have fled Ukraine since the invasion, most of them to Poland, in Europe's worst refugee crisis since World War II.

As Russia widens its bombardment, Zelensky's pleas for help have grown increasingly desperate.

Washington and its EU allies have sent funds and military aid to Ukraine and taken action against Russia's economy and oligarchs. A cultural and sporting boycott has further isolated Moscow.

On Saturday, US President Joe Biden authorised up to $200 million in weapons and other aid to Ukraine.

In Irpin, a Ukrainian soldier who gave his name only as Viktor showed off his British anti-tank missile system and the twisted remains of a Russian vehicle it destroyed.

"I want to say a big thank you to our British comrades," he said.

As international sanctions against Moscow have steadily tightened, crippling Russia's economy, the country's space agency Roscosmos warned Saturday that the International Space Station could crash if Russian spacecraft serving the ISS are affected.

- 'Cinders in his lungs' -

The situation in Mariupol remains "desperate", according to Doctors Without Borders, with no water or heating -- and food supplies dwindling.

"Hundreds of thousands of people... are for all intents and purposes besieged," Stephen Cornish, one of those heading the medical charity's Ukraine operation, told AFP.

Prime Minister Denys Shmyhal said it would be necessary to restructure the country's economy given the lack of supplies throughout, and urged people in areas with no active combat to return to work.

In the southern city of Mykolaiv, meanwhile, an AFP reporter said a hospital there came under fire.

Mykolaiv, which lies on the road to the strategic port city of Odessa, has been under attack for days.

"They shot at the civilian areas, without any military objective," said the hospital's head, Dmytro Lagochev.

In Kharkiv, in the east, doctors at a hospital described spending two days pumping ash from the stomach of an eight-year-old whose home was struck by a Russian missile.

"He still has cinders in his lungs," Dima Kasyanov's doctor told AFP.

- 1,300 Ukrainian troops -
Facing growing international condemnation, Putin on Saturday sought to turn the tables, slamming Kyiv for what he described as the "flagrant violation" of international humanitarian law and accusing Ukraine's army of executing dissenters and using civilians as hostages.

The French presidency denounced his accusations as "lies".

In a stream of video messages, Zelensky has urged Ukrainians to keep fighting and demanded his country's allies do more.

On Saturday, he said Moscow was suffering "enormous losses", before giving Ukraine's first toll of around 1,300 troops killed so far.

US estimates put Russian fatalities at 2,000 to 4,000 while Moscow's only official toll, announced last week, said 498 Russian troops had been killed.

Latest developments

Here are the latest developments in Russia's war in Ukraine:

- Russia encircling Kyiv -
Russian forces advance ever closer to the capital from the north, west and northeast. Russian strikes also destroy an airport in the town of Vasylkiv, south of the city.

Presidential adviser Mykhailo Podolyak says Kiev is already effectively "under siege" as the army and volunteers prepare to defend the city, street by street.

- US authorises $200m military aid -
US President Joe Biden authorises $200 million in additional military equipment for Ukraine. Washington has already authorised $350 million of military equipment -- the largest such package in US history.

- Ukraine says Russians shot 7 evacuees -
Russian troops shot at a group of women and children evacuees leaving a village near Kyiv, killing seven, one of them a child, Ukraine's military intelligence service says.

- 1,300 Ukrainian troops killed -
"Around 1,300" Ukrainian troops have been killed since the start of the invasion, President Volodymyr Zelensky says, the first time he has given a toll for Ukraine's forces. Russia said on March 2 it had lost 498 soldiers, but Zelensky says it is closer to 12,000.

At least 579 Ukrainian civilians have been killed, the UN says.

- French, German appeal to end siege -
The leaders of France and Germany urge Russian President Vladimir Putin in a call to end the more than week-long siege of the port of Mariupol. Residents have been trapped without running water and power for 11 days and there are reports of people starving. 

The mayor says more than 1,500 civilians have been killed. Corpses are lying uncollected in the streets.

- Mayor abducted -
Ukrainian lawmakers say Russian soldiers have abducted the mayor of the southern city of Melitopol, Ivan Fedorov.  

A senior official publishes a video showing soldiers escorting a man from a building, his head apparently covered in a black bag. Zelensky asks France and Germany to help secure his release.

- Zelensky says Moscow taking different approach -
In a media briefing, Zelensky says the approach being taken by Moscow is in contrast to earlier talks at which Moscow only "issued ultimatums" and that he is "happy to have a signal from Russia" after President Vladimir Putin said he saw "some positive shifts" in their dialogue.

- Russian engineers at nuclear plant -
Russian engineers arrive to measure radiation at a Ukrainian nuclear plant, the seizure of which has sparked international alarm, officials say.

- Cancer hospital hit - 
A cancer treatment hospital and an eye clinic suffer damage in bombardments of the city of Mykolaiv, near the strategic Black Sea port of Odessa, a few days after a maternity hospital in Mariupol was shelled.

Ukraine claims a mosque where about 80 civilians, including some Turkish nationals, were sheltering in Mariupol was shelled but a person staying there denies the report, saying the bomb fell 700 metres away.

- Amsterdam's Orthodox clergy split from Moscow Patriarch -
The clergy of the Russian Orthodox Church in the Dutch city of Amsterdam, announce they are splitting from the Moscow church because of threats to them over Russia's invasion of Ukraine.

- Space station in jeopardy - 
The head of Russian space agency Roscosmos warns that Western sanctions could cause the International Space Station to crash.

Dmitry Rogozin says the sanctions are affecting supplies to the Russian part of the station, which corrects the station's orbit.

- Abramovich disqualified -
The Premier League board disqualifies Roman Abramovich as a director of Chelsea Football Club after the Russian billionaire was placed under Western sanctions.

Abramovich has already announced his intention to sell the club.

- No 'World War III' -
US President Joe Biden again rules out any direct intervention by the United States to halt Russia's invasion of Ukraine, warning that such conflict pitting the NATO alliance against the Kremlin "is World War III".

- Bio-weapons claims rejected -
Western countries reject Russian claims that the United States and Ukraine researched using bats to conduct biological warfare. Britain's ambassador to the UN says the claims are "utter nonsense".

- 2.5 million flee -
More than two and a half million people have fled the "senseless war" in Ukraine, the UN says -- more than half to Poland.