Ukraine accused Russia Thursday of bombing a theatre that was sheltering more than 1,000 civilians in the city of Mariupol, after US President Joe Biden branded Vladimir Putin a "war criminal".

The latest assaults on civilians across Ukraine came as President Volodymyr Zelensky made a searing appeal for help to the US, which responded by pledging $1 billion in new weapons to fight Russia's invading army.

Officials across Ukraine are struggling to count the civilian dead -- with authorities saying 103 children have been killed since the invasion began -- who have been targeted in homes, hospitals, ambulances and food queues.

In the port city of Mariupol -- where more than 2,000 people have died so far -- a Russian bomb hit the Drama Theatre, which city council officials said had been housing over 1,000 people. 

"The only word to describe what has happened today is genocide, genocide of our nation, our Ukrainian people," the city's mayor Vadim Boychenko said in a video message on Telegram.

"We have difficulty understanding all of this, we refuse to believe, we want to close our eyes and forget the nightmare that happened today," he said. 

Satellite images of the theatre on March 14 shared by private satellite company Maxar showed the words "children" clearly etched out in the ground in Russian on either side of the building.

Officials posted a photo of the building, whose middle part was completely destroyed, with thick white smoke rising from the rubble after they said a bomb was dropped from an airplane.

"It is impossible to find words to describe the level of cynicism and cruelty, with which Russian invaders are destroying peaceful residents of a Ukrainian city by the sea," an official statement read.

Russia's defence ministry denied it had targeted the theatre, instead claiming that the building had been mined and blown up by members of Ukraine's far-right Azov Battalion.

So far the destruction that has marked other cities has been halted outside the capital Kyiv, which has been emptied of around half of its 3.5 million people.

But dull booms echoed across the capital's deserted streets Wednesday, with only an occasional vehicle passing through sandbagged checkpoints, and very few permits granted to break its latest curfew.

-'War criminal' - 

In an address to the US Congress, Zelensky invoked Pearl Harbor, the 9/11 attacks and Martin Luther King Jr as he showed lawmakers a video of the wrenching effect of three weeks of Russian attacks.

Zelensky, dressed in military green, demanded Washington and its NATO allies impose a no-fly zone, so that "Russia would not be able to terrorize our free cities."

Switching to English, Zelensky addressed Biden directly, saying: "I wish you to be the leader of the world. Being the leader of the world means to be the leader of peace."

Biden and NATO have resisted Zelensky's pleas for direct involvement against nuclear-armed Russia, warning it could lead to World War Three -- though the Ukrainian leader told NBC that "may have already started."

But on Wednesday Biden announced the United States' latest package of new weapons aid to Ukraine added up to $1 billion and that the US would help the ex-Soviet state acquire longer-range anti-aircraft weapons.

The US president also stepped up his condemnation of the Russian leader, describing him as a "war criminal."

The Kremlin called the comment "unacceptable and unforgivable on the part of the head of a state whose bombs have killed hundreds of thousands of people around the world."

Britain's diplomatic mission to the UN also tweeted that Russia is committing "war crimes and targeting civilians" in Ukraine, after the British government requested an emergency UN Security Council meeting over the deteriorating humanitarian situation there.

"Russia's illegal war on Ukraine is a threat to us all," it posted Wednesday, saying the request was made with the US, France, Albania, Norway and Ireland.

Putin at a televised government meeting insisted the invasion was "developing successfully," adding "we will not allow Ukraine to serve as a springboard for aggressive actions against Russia."

As his government accelerates a crackdown that saw at least a dozen media websites blocked Wednesday, Putin claimed that the West sought to divide Russian society, railing against a "fifth column" that was "mentally" in the West. 

"Russian people will always be able to distinguish true patriots from traitors and just spit them out like a fly that accidentally flew into their mouth," he said. 

He also condemned western sanctions against his regime that have pushed Russia close to a default on its foreign debts as "economic blitzkrieg".

- 'Teetering on the brink' -

As the civilian toll in Ukraine climbed, the World Health Organization said that healthcare facilities and personnel were being attacked at an unprecedented rate.

"We've never seen globally... this rate of attacks on healthcare," the WHO's emergencies director Michael Ryan told reporters, warning that "this crisis is reaching a point where the health system in Ukraine is teetering on the brink".

The UN health agency has verified 43 attacks on health facilities, ambulances and health personnel in Ukraine since the Russian invasion began on February 24, killing 12 people and injuring 34. 

And the conflict has already sent more than three million Ukrainians fleeing across the border, many of them women and children, 103 of whom have been killed since the invasion began, authorities have said.

Kremlin spokesman Dmitry Peskov said a "compromise" outcome would centre on Ukraine becoming a neutral state comparable to Sweden and Austria.

But Zelensky's office gave the idea short shrift -- and many Ukrainians themselves remained defiant.

Retired tennis player Alexandr Dolgopolov went home to Kyiv to take up arms and defend his native city. 

"Used to be rackets and strings, now this," the 33-year-old wrote on Instagram alongside a photo of a rifle, helmet and flak jacket.

The mayor of Ukraine's southern city of Melitopol was released days after Kyiv said he was abducted by Russian forces.

"Thank you for not abandoning me," Ivan Fedorov told Zelensky, according to a video of their phone call posted on Telegram. 

"I will need one or two days to recover and then I will be at your disposal to contribute to our victory."

Latest developments

Here are the latest developments in the war in Ukraine:

- Ten killed in bread queue -

Ukraine says 10 people were killed after coming under fire while queueing for bread in the northern city of Chernigiv.

- Mayor freed -

The mayor of Melitopol is freed almost a week after he was reportedly abducted by Russian forces. He was reportedly released as part of an exchange for several young Russian conscripts captured by Ukraine. 

- Zelensky speaks to US Congress - 

In an impassioned video address to the US Congress, Ukrainian President Volodymyr Zelensky likens Russia's invasion to 9/11 and the attack on Pearl Harbor which drew the US into World War II.

After pleading with allies to impose a no-fly zone and showing a harrowing montage of the devastation wrought on Ukrainian civilians, US lawmakers give him a standing ovation.

- US weapons aid -

Shortly after, Biden announces a massive package of military aid for Ukraine, including S-300 missile defence systems, 100 Switchblade "kamikaze" drones and thousands more anti-aircraft and anti-tank missiles.

- No NATO forces for Ukraine -

NATO chief Jens Stoltenberg says the alliance is not planning to send forces to Ukraine, after Poland called for it to deploy a peacekeeping mission there.

- Putin doubles down -

In meeting with regional officials Putin repeats claims that the "pro-Nazi Kiev regime" is carrying out "genocide" and trying to obtain "weapons of mass destruction" including developing "military biological programmes" with the "financial support of the Pentagon."

- Ukraine sets out demands -

After days of cautious optimism about peace talks between Russia and Ukraine, Zelensky reiterates his demands: an end to the war, security guarantees and a return to sovereignty and territorial integrity.

- US-Russia call -

US National Security Adviser Jake Sullivan and General Nikolay Patrushev, secretary of the Russian Security Council, hold the first high-level contact between the countries since the invasion began. 

- Payment day for Russia -

Russia is due to make a $117 million (107 million euros) interest payment on its foreign debt. It says it will service its dollar-denominated debt in rubles -- which could result in a default.

- Putin announces economic relief -

Putin says the West's "economic blitzkrieg" against Moscow has failed, but admits the situation is "not easy" for Russians and announces an increase in all social payments.

- News sites blocked -

Russia's media regulator Roskomnadzor blocks access to at least 32 websites, ramping up a crackdown on the media launched after the start of the war.

Media affected include the BBC, the award-winning investigative website Bellingcat and regional site Permdaily.ru.

- Out of Council of Europe -

Russia ceases to be a member of the Council of Europe after over a quarter of a century of membership in the pan-European rights body.

- China's ambassador -

China's ambassador tells Ukrainian officials in Lviv that "China is a friendly country" that "will forever remain a good force for Ukraine," according to Ukrainian accounts of the meeting.