Ukraine has reacted with scepticism to Russia’s promise in face-to-face peace talks to scale down military operations around Kyiv and another city as some Western countries expect Moscow to intensify its offensive in other parts of the country.

Negotiators from both nations met in an Istanbul palace on Wednesday (Australian time), more than a month into the largest attack on a European nation since World War II.

Moscow’s invasion of its neighbour has already killed or injured thousands, forced nearly four million to flee abroad and pummelled Russia’s economy with sanctions.

The invasion has been halted on most fronts by stiff resistance from Ukrainian forces, who have recaptured territory even as civilians are trapped in besieged cities.

“In order to increase mutual trust and create the necessary conditions for further negotiations and achieving the ultimate goal of agreeing and signing [an] agreement, a decision was made to radically, by a large margin, reduce military activity in the Kyiv and Chernihiv directions,” Russian Deputy Defence Minister Alexander Fomin said.

He made no mention of other areas where there has been heavy fighting, including around the battered city of Mariupol in the south-east, Sumy and Kharkiv in the east and Kherson and Mykolaiv in the south.

“Ukrainians are not naive people,” Ukrainian President Volodymyr Zelensky said later.

“Ukrainians have already learned during these 34 days of invasion, and over the past eight years of the war in Donbass, that the only thing they can trust is a concrete result.”

The general staff of the Ukrainian armed forces said Russia’s promise to curtail military operations in some areas was “probably a rotation of individual units and aims to mislead”.

Russia had started moving handfuls of troops away from positions around Kyiv in a move that was more of a repositioning than a retreat or a withdrawal, the Pentagon said.

“We all should be prepared to watch for a major offensive against other areas of Ukraine,” spokesman John Kirby said.

“It does not mean that the threat to Kyiv is over.”

Britain’s Ministry of Defence said: “It is highly likely that Russia will seek to divert combat power from the north to their offensive in the Donetsk and Luhansk regions in the east.”

Russia calls its assault a “special operation” to disarm and “denazify” Ukraine. The West says it launched an unprovoked invasion.

Some analysts noted Russia’s promise to reduce fighting mostly covered areas where it had been losing ground.

Under Ukraine’s proposals, Kyiv would agree not to join alliances or host bases of foreign troops, but would have security guaranteed in terms similar to Article 5, the collective defence clause of the NATO alliance.

Negotiators named Israel and NATO members Canada, Poland and Turkey as countries that might give such guarantees. Russia, the US, Britain, Germany and Italy could also be involved.

The proposals, which would require a referendum in Ukraine, mention a 15-year consultation period on the status of Crimea, which was annexed by Russia in 2014.

The fate of the south-eastern Donbass region, which Russia demands Ukraine cede to separatists, would be discussed by the Ukrainian and Russian leaders.

Kyiv’s proposals also included one that Moscow would not oppose Ukraine joining the European Union, Russia’s lead negotiator Vladimir Medinsky said. Russia has previously opposed Ukrainian membership of the EU and especially of NATO.

Mr Medinsky said Russia’s delegation would study and present the proposals to President Vladimir Putin.

US President Joe Biden discussed with allies more financial aid of up to $US500 million ($670 million) for Ukraine, a source familiar with the situation told Reuters.

However, reports of attacks continue.

A shell hit a temporary Russian military camp near the border with Ukraine late on Tuesday, Tass news agency said, with preliminary data showing it had been fired from the Ukrainian side. If confirmed, it would be the first direct hit on Russian territory from Ukraine since the invasion began.

In Ukraine’s besieged port Mariupol, thousands of civilians may have died, the head of the United Nations human rights mission in the country told Reuters.

Elsewhere, Ukrainian forces have made advances, recapturing territory from Russian troops on the outskirts of Kyiv, in the north-east and in the south.

-with AAP