Russian and Ukrainian negotiators began face-to-face talks in Istanbul on Tuesday, the official Turkish news agency said, with host Turkish President Recep Tayyip Erdogan urging them to "put an end to this tragedy".

The face-to-face talks at the Dolmabahce palace in Istanbul are aimed at trying to end a war has killed an estimated 20,000 people and forced more than 10 million from their homes.

"The two parties have legitimate concerns. It's possible to reach a solution acceptable to the international community," Erdogan said.

"It's up to the two parties to put an end to this tragedy," he insisted, adding that the "extension of the conflict is in no one's interest".

"The whole world is waiting for good news from you," Erdogan continued.

Turkish Foreign Minister Mevlut Cavusoglu was also due to meet the Ukrainian and Russian delegations on Tuesday.

Turkey previously hosted on March 10 the first meeting between the Ukrainian and Russian foreign ministers following Russia's invasion of Ukraine on February 24.

Those talks in the southern city of Antalya failed to produce a ceasefire or make any other visible progress. 

On Monday evening, Erdogan said his country was the only one, since Russia's annexation of Crimea in 2014, to have made genuine efforts to find a solution to the crisis through dialogue, negotiation and an agreement.

Turkey, which shares a Black Sea coast with both Russia and Ukraine, is seeking to maintain good relations with both and has offered to mediate since the start of the war.

Ankara is a traditional ally of Kyiv's and has supplied the country with Bayraktar drones, which Ukraine has deployed in the conflict. But it is also seeking to stay on good terms with Russia, on which Turkey depends heavily for gas imports and tourism revenues. 

Turkey is also working with France and Greece on a "humanitarian operation" to evacuate people from the devastated Ukrainian port city of Mariupol, which has been pounded by Russian forces. 

Ukraine retakes Kyiv suburb

Ukrainian forces recaptured a key Kyiv suburb and desperately clung onto control of the besieged city of Mariupol, as negotiators prepared to meet Russian counterparts for face-to-face talks in Istanbul Tuesday.

Troops "liberated" the suburban town of Irpin, Interior Minister Denys Monastyrsky said, wresting a key gateway to the capital's northwest from Russian control. 

AFP journalists witnessed continued heavy shelling in the area and encountered fleeing residents, who described hellish scenes of bombs raining from the sky and people killed in cold blood while trying to escape.

"We saw those cars which tried to get out on their own, they were crushed by tanks, with people inside," said 55-year-old Roman Molchanov, his voice cracking with emotion.

His sister added that the "Russian orcs" had "shot dead people sitting in their cars."

Western experts described the loss of Irpin as a significant setback for Russian forces, who are still trying to regroup and after a failed first attempt to encircle the capital.

It is now more than a month since Russian President Vladimir Putin's tanks rolled into Ukraine, hoping to cripple or oust the democratic government in Kyiv. 

The fighting has already killed an estimated 20,000 people and forced more than 10 million from their homes.

The prospects of a peaceful end to the war -- or an imminent victory for either side -- appear faint.

Ukrainian and Russian negotiators will resume peace talks on Tuesday, under the shadow of shock allegations that delegates were poisoned at a previous round of negotiations.

Russian oligarch Roman Abramovich and Ukrainian negotiators were said to have been targeted earlier this month, the Wall Street Journal reported, citing people familiar with the matter.

Abramovich -- a billionaire businessman under Western sanctions -- and the negotiators reportedly developed symptoms including red eyes and peeling skin, though they later recovered.

Ukraine played down the allegations and Foreign Minister Dmytro Kuleba said the Istanbul talks would focus on easing the humanitarian situation, and sounded a note of scepticism about the hopes for success.

"If we see that the mood has changed and they are ready for a serious, substantive conversation and balanced arrangements, then things will move forward," he said.

"If it is a repetition of their propaganda," then, he said, talks will again fail.

Putin has demanded the "demilitarisation and denazification of Ukraine", as well as the imposition of neutral status and recognition of the Donbas and Crimea as no longer part of Ukraine.

Kubela indicated there was little room for agreement there: "We do not trade people, land and sovereignty. Our position is concrete."

- Fighting rages -

On the battlefield, both sides appear determined to press where they can. 

Ukrainian officials still believe that Russia wants to take the capital Kyiv, dismissing suggestions the Kremlin is focused on the eastern Donbas region.

Capturing "Kyiv is essentially a captured Ukraine, and this is their goal," said deputy defence minister Ganna Malyar, insisting Russia was still "trying to break through the corridor around Kyiv and block transport routes." 

On Monday Russian attacks near Kyiv cut power to more than 80,000 homes, officials said, underscoring the continued peril facing the capital.

While Ukraine's forces are counterattacking in the north, they are struggling to retain control of the southern port city of Mariupol.

Russian forces have encircled the city and have embarked on a steady and indiscriminate bombardment, trapping an estimated 160,000 people with little food, water or medicine.

At least 5,000 people have already died, according to one senior Ukrainian official who estimated the real toll may be closer to 10,000 when all the bodies are collected.

"The burials stopped 10 days ago because of continued shelling," Tetyana Lomakina, a presidential adviser now in charge of humanitarian corridors, told AFP by phone Monday.

Local lawmaker Kateryna Sukhomlynova told AFP that unburied bodies line streets and residents cowering in basement shelters have been forced to eat snow to stay hydrated. 

Ukraine's foreign ministry called the situation "catastrophic," saying Russia's assault from land, sea and air had turned a city once home to 450,000 people "into dust".

France, Greece and Turkey are hoping to launch a mass evacuation of civilians from Mariupol within days, according to French President Emmanuel Macron, who is seeking agreement from Putin.

But as the ground war has stalled and Russian casualties have mounted. Moscow appears to have turned to ever-more brutal tactics.

Western powers say they have seen evidence of war crimes, which are already being investigated by the International Criminal Court.

On Monday, Ukraine's prosecutor general, Iryna Venediktova, said there was proof that Russian forces have used banned cluster bombs in the southern Odessa and Kherson areas.

And Britain's defence ministry said that private Russian military firm Wagner Group has headed to eastern Ukraine, where "they are expected to deploy more than 1,000 mercenaries, including senior leaders of the organisation, to undertake combat operations."

Reputed to be close to Putin, the Wagner Group and its mercenaries are suspected of widespread abuses in Mali, Libya and Syria.

US President Joe Biden's has expressed his "moral outrage" at the conduct of the war, and ruffled feathers by suggesting Putin "cannot remain in power".

He has since denied seeking regime change and swatted away concern that his remarks would ratchet up tensions with Putin. 

"I don't care what he thinks," Biden said as he proposed $6.9 billion in funding to Ukraine and NATO, and another $1 billion to help counter Moscow's influence.

Latest developments

Here are the latest developments in the war in Ukraine:

- 5,000 dead in Mariupol -

A senior Ukrainian official says at least 5,000 people have been buried in the besieged southern city of Mariupol since the invasion began, but that bodies have gone uncollected for the past 10 days amid continuing Russian shelling.

The foreign ministry describes the humanitarian situation in the city as "catastrophic".

- Poisoning report -

Russian oligarch Roman Abramovich and Ukrainian negotiators were targets of a suspected poison attack at peace talks earlier this month, the Wall Street Journal reports, citing people familiar with the matter.

Ukrainian officials decline to confirm the incident, advising people to follow "only the official information", and warning about "various conspiracy theories."

Abramovich and the negotiators reportedly developed symptoms including red eyes and peeling skin, though they later recovered.

- Wagner group in Ukraine: UK -

Britain's defence ministry says Russia's Wagner Group has deployed its mercenaries to eastern Ukraine, adding that more than 1,000 fighters were expected to participate in the war after Russian setbacks.

- UN seeking humanitarian ceasefire -

The United Nations is seeking a humanitarian ceasefire, chief Antonio Guterres says, announcing he has tasked UN humanitarian chief Martin Griffiths to "explore... possible agreements" with both sides on trips to Moscow and Kyiv.

- US budget includes Ukraine aid -

US President Joe Biden unveils a budget proposal with a four-percent increase in defence spending and a $6.9 billion infusion of funding for Ukraine and NATO, with another $1 billion allocated to Washington's efforts to counter Moscow's influence.

Biden meanwhile is again forced to defend his remarks that Putin "cannot stay in power", saying the comments were not a policy change but expressed his "moral outrage."

- Fierce fighting around Kyiv -

Russian attacks near Kyiv cut power to more than 80,000 homes, officials say, underscoring the peril facing the capital despite an apparent retreat in Moscow's war aims to focus on eastern Ukraine. 

"The enemy is trying to break through the corridor around Kyiv and block transport routes," Ukraine's deputy defence minister Ganna Malyar says.

- $565 billion hit -

Ukraine's Economy Minister Yulia Svyrydenko says on Facebook that invasion has cost her country an estimated $564.9 billion, including immediate damage and expected hits to trade and economic activity.

- Top newspaper silenced -

Russia's top independent newspaper Novaya Gazeta, whose chief editor was last year awarded the Nobel Peace Prize, suspends publication until the end of the war in Ukraine.

The paper, which has fallen foul of Russian censors for its reporting on the war, says it took the decision "to save us for each other".

- 'Unfriendly' states -

Russia says it is preparing to restrict entry to the country for nationals of "unfriendly" countries, including Britain, EU members and the United States, in retaliation for Western sanctions on the country.

Meanwhile rival brewers Heineken and Carlsberg become the latest Western firms to pull out of Russia.