Ukraine on Wednesday accused Russia of shelling Chernigiv in northern Ukraine despite promising to scale back military activity around the city and the capital Kyiv after more than a month of war.

Ukraine and Western powers have cast doubt on Russia's pledge, made during face-to-face talks between Russian and Ukrainian delegations in Istanbul on Tuesday.

"The enemy has demonstrated its 'decrease in activity' in the Chernigiv region with strikes on Nizhyn, including air strikes," regional governor Vyacheslav Chaus wrote on social media.

"Chernigiv was shelled all night," he said.

AFP reporters on Wednesday could also hear frequent explosions coming from the direction of the suburban town of Irpin to the northwest of Kyiv.

Ukrainian forces have said they are in control of the town but emergency services said it was still too dangerous for civilians to access. 

"The area might be within mortar range so it is still dangerous," said Petro Kyseliov, acting head of the State Emergency Service of Ukraine in Kyiv.

Russian officials had pledged to "radically" reduce attacks because of progress in negotiations on "the neutrality and non-nuclear status" of Ukraine -- two central concerns for Moscow.

Both sides called the Istanbul meeting "meaningful" and "positive", raising hopes of a deescalation.

Thousands of people have died and millions been displaced since Russia launched the invasion of its pro-Western neighbour on February 24.

But the Pentagon said Russia had merely repositioned a "small number" of forces near Kyiv, and could be preparing a "major offensive" elsewhere.

The "vast majority" of Russian forces around Kyiv remained in place, said Pentagon spokesman John Kirby.

"Russia has failed in its objective of capturing Kyiv," the Pentagon spokesman added, but "it does not mean that the threat to Kyiv is over".

Ukraine's military also warned the withdrawal of Russian troops around Kyiv and Chernigiv "is probably a rotation of individual units and aims to mislead".

We'll see

Ukrainian President Volodymyr Zelensky described "positive" signs from the Istanbul talks, which are expected to continue via video, but said there were no plans to let down defences.

The signals "do not drown out the explosions or Russian shells", he said in a late Tuesday video address, urging no talk of lifting sanctions on Moscow until the war is over.

Ukraine's Western allies said they had no plans to ease measures taken to punish Russia for the invasion.

"We'll see if they follow through on what they're suggesting," US President Joe Biden said after speaking with the leaders of Britain, France, Germany and Italy, who vowed no let-up in sanctions.

On Wednesday, Poland urged the European Union to impose a tax on Russian hydrocarbon imports while Germany raised the alarm level under its emergency gas plan over fears Russia could cut supplies to countries that refuse its demand to be paid in rubles.

Washington meanwhile warned citizens that Moscow could "single out and detain" them in Russia, repeating calls for Americans in the country to leave immediately.

Still, the talks in Istanbul marked the first sign of progress in discussions to end the conflict, with Kyiv's negotiator David Arakhamia saying there were "sufficient" conditions for Zelensky to meet Russian President Vladimir Putin.

Moscow had already signalled last weekend that it was dialling back its war goals, focusing its military resources on capturing the eastern Donbas region.

In recent days, Ukraine's fighters have recaptured territory including Irpin.

"The Russians were slowly backed off from Irpin... So now the priority task is to go there and bring back the bodies of those killed," Kyseliov said.

"The bodies are still lying around the streets, they are starting to decay and smell and dogs and other animals are starting to eat them," he said.

Death everywhere

Some 20,000 people are believed to have been killed in the conflict so far, according to Zelensky, though the number of casualties could not be independently verified.

On Tuesday a Russian missile strike on the southern town of Mykolaiv left at least 12 dead and 33 wounded, Ukrainian officials said.

There was also no progress for the estimated 160,000 people still trapped with little food, water or medicine in the devastated southern port city of Mariupol. 

Russian forces have encircled the city and their steady and indiscriminate bombardment has killed at least 5,000 people, but possibly as many as 10,000, according to one senior Ukrainian official.

France, Greece and Turkey have been trying to organise a mass evacuation of civilians from the city, but talks between French President Emmanuel Macron and Putin ended Tuesday without a deal.

Aid groups have called regularly for access to Mariupol, decrying hellish conditions, and Ukrainian officials have accused Russian troops of forcibly deporting residents to Russia.

Civilians who have managed to escape Mariupol describe a place with "death everywhere".

"We buried our neighbours, we saw death everywhere and even my children saw it," said Mariia Tsymmerman, who fled to Zaporizhzhia two weeks ago but is now making the perilous journey back to deliver supplies and help others leave.

"I know a woman who killed her own dog to feed her children," she said.