Ukrainian and Russian negotiators held face-to-face talks in Istanbul on Tuesday as Ukraine resumed evacuations from territory occupied by Russian forces and clung on in the besieged city of Mariupol.

The talks were taking place with Turkish President Recep Tayyip Erdogan in attendance and under the shadow of shock allegations that delegates were poisoned at a previous round of negotiations.

Erdogan called on the delegations to "put an end to this tragedy," saying both Russia and Ukraine both have "legitimate concerns" ahead of the meeting at the Dolmabahce Palace.

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 forced more than 10 million from their homes and according to Ukrainian President Volodymyr Zelensky has killed an estimated 20,000 people.

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

On the edge of the suburban town of Irpin to the northwest of Kyiv, AFP reporters on Tuesday heard the sound of sporadic shell fire, a day after Ukrainian forces said they had recaptured the town.

"In my opinion, maybe some 70-80 percent (of the town) is free, while the outskirts are taken" by Russians, said Irpin resident Roman Kovalevskyi, 48, who was cycling out of the town to get supplies from Kyiv.

Negotiators poisoned?

Ukraine also announced that evacuations from several areas under Russian control in the south of the country were being resumed on Tuesday, a day after Ukrainian officials suspended them saying they feared Russian "provocations" along the humanitarian corridors.

The talks in Istanbul come after a report in the Wall Street Journal said Russian oligarch Roman Abramovich and Ukrainian negotiators were poisoned earlier this month after talks in Kyiv.

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

Zelensky has said his government received an offer of support from Abramovich, who has long-standing links to Putin.

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 skepticism 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," he said, talks will again fail.

Abramovich was present at the talks in Istanbul on Tuesday, according to a photograph released by the Turkish presidency.

Putin has demanded the "demilitarization 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.

'Catastrophic' situation

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

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 defense 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.

Mariupol evacuations

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.

In Mykolaiv, another key city in the south, the local governor said that a Russian strike hit the regional administrative building and eight civilians and three soldiers were missing.

As 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.

US President Joe Biden has expressed his "moral outrage" at the conduct of the war, and ruffled feathers over the weekend 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 on Monday as he proposed $6.9 billion in funding to Ukraine and NATO, and another $1 billion to help counter Moscow's influence.