Ukraine and Western allies waited Wednesday for signs Moscow was "radically" reducing military activity around Kyiv, as promised in peace talks, with scepticism high after Washington warned Russian troops were being repositioned and not withdrawn.

Both sides called talks in Istanbul "meaningful" and "positive", in sharp contrast to previous rounds of discussions, raising hopes after more than a month of war that has killed thousands and displaced millions.

Russia's deputy defence minister Alexander Fomin said there was progress on "the neutrality and non-nuclear status" of Ukraine -- two central Russian concerns.

And he said Russia would "radically, by several times reduce the military activity" around Kyiv and the northern city of Chernigiv.

But the pledge was met with scepticism in Ukraine and Western capitals, with the Pentagon saying Russia had merely repositioned a "small number" of forces near Kyiv, but could be preparing a "major offensive" elsewhere.

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

"We've only seen a small number begin to move away from Kyiv, mostly to the north.

"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".

On the ground overnight, air raid sirens sounded several times in the capital and continued into the morning.

- 'We'll see if they follow through' -

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.

Belgium, Ireland and the Netherlands on Tuesday announced 42 Russian diplomats would be told to leave, with Moscow in turn expelling 10 diplomats from Estonia, Latvia and Lithuania.

Still, the face-to-face 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.

And optimism over the apparent progress sent European and US stock markets up, while oil prices fell by five percent as supply fears eased, and the ruble surged 10 percent against the dollar.

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 the strategic Kyiv suburb of Irpin, and Britain's defence ministry said overnight "it is almost certain that the Russian offensive has failed in its objective to encircle Kyiv".

"It is highly likely that Russia will seek to divert combat power from the north to their offensive in the Donetsk and Luhansk regions (of Donbas) in the east," the ministry 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.

"I was having breakfast in my apartment," Donald, 69, a retired Canadian postal worker with Ukrainian residency told AFP. "I heard a whoosh, then a boom and my windows rattled."

Another local resident, Viktor Gaivonenko, who was helping clean up the debris, said: "Putin is a bastard. That's all there is to it."

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.

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.

 US advises that Moscow may 'detain' Americans in Russia: State Dept

The US State Department issued a travel advisory on Tuesday warning that Moscow "may single out and detain US citizens in Russia" and repeating earlier warnings for Americans not to travel to the country.

The warning was "due to the unprovoked and unjustified invasion of Ukraine by Russian military forces" as well as the potential for harassment of US citizens by Russian authorities, the travel advisory said, repeating calls for Americans traveling or living in Russia to leave "immediately."

It comes days after a US diplomat was able to visit detained basketball star Brittney Griner, who has been held in Russian custody for more than a month. 

Griner, a two-time Olympic gold medalist and WNBA champion, was detained at a Moscow airport on February 17 on charges of carrying vape cartridges that contained cannabis oil in her luggage.

After being apprehended, Griner was immediately placed in a detention center, and last week a Russian court extended her arrest until May 19. She faces up to 10 years in prison.

The arrest came as relations between Moscow and the West hit rock bottom over Russia's invasion of Ukraine.

The administration of US President Joe Biden has led the Western response to the invasion, levelling unprecedented sanctions against Moscow and funnelling weapons and aid to Kyiv.

Latest developments

Here are the latest developments in the war in Ukraine:

- Putin, Zelensky meeting possible: Russia, Ukraine -

Ukraine's top negotiator says enough progress had been made at the talks in Turkey to enable a meeting between the presidents of the two countries.

"The results of today's meeting are sufficient for a meeting at the leaders' level," David Arakhamia says.

- Biden, Europe urge caution -

The leaders of the United States, UK, France, Germany and Italy urge the West not to drop its guard against Russia.

"They agreed there could be no relaxation of Western resolve until the horror inflicted on Ukraine is over," they said, according to a read-out from UK Prime Minister Boris Johnson's office.

They also agree during telephone talks to "continue raising costs on Russia", the White House says, with President Joe Biden saying the West needs to see if Russians "follow through" on de-escalation.

- Putin stands firm on Mariupol -

Russian President Vladimir Putin tells French counterpart Emmanuel Macron Ukrainian "nationalists" in the embattled port city of Mariupol must lay down their arms "in order to resolve the difficult humanitarian situation", the Kremlin says.

The French presidency says the conditions for carrying out a humanitarian operation sought by France to help citizens in the besieged port are not met "at this stage."

Earlier, Ukrainian President Volodymyr Zelensky called the month-long Russian siege of the city, where at least 5,000 people have died since the invasion began according to Ukrainian estimates, a "crime against humanity".

- Ukraine seeks long-term security guarantees -

Ukraine says it is prepared to become neutral in return for an international agreement guaranteeing its security.

Ukrainian negotiator Arakhamia says the guarantee would resemble Article 5 of NATO's founding treaty, which requires member states to come to each other's aid in case of an attack.

- NATO no, EU yes -

Ukraine reiterates that it will not join "any military-political alliance" -- a reference to NATO -- or host foreign military bases on its soil, key demands of Russia.

Kyiv insists, however, that Ukraine be allowed to seek EU membership. It proposes to defer the thorny question of the future status of Crimea and the breakaway Donbas region.

- Ruble and stocks up, oil down -

Oil prices dive and European stock markets close higher on the positive news from the talks.

The ruble surges by more than 10 percent against the dollar while oil falls by more than five percent.

- Seven killed in Mykolaiv strike -

At least seven people are killed in a Russian strike on a regional government building in the key southern port of Mykolaiv, Zelensky tells Danish MPs.

- Evacuations resume - 

Ukraine says it is resuming the evacuation of civilians from war-scarred regions after a one-day pause over what it called the threat of Russian attacks.

"Three humanitarian corridors were agreed for today," Deputy Prime Minister Iryna Vereshchuk says on Telegram.

- EU countries expel diplomats -

EU countries Belgium, the Netherlands, Ireland and the Czech Republic announce the expulsion of dozens of Russian diplomats suspected of spying, in coordinated action taken in the shadow of Moscow's war in Ukraine.