Some Russian units suffering heavy losses in Ukraine had been forced to return home and to neighbouring Belarus, British military intelligence said a day after Russia promised to scale down military operations around Kyiv and another city.

Ukrainian President Volodymyr Zelensky reacted with scepticism to the offer made by Russia during negotiations in Istanbul aimed at de-escalating a conflict now in its fifth week.

His forces have halted the invasion on most fronts, and some analysts noted that Russia's promise to reduce fighting mostly covered areas where it has been losing ground, even as civilians remained trapped in besieged cities in the south and east.

Heavy losses and the withdrawal of some troops was impacting Russian operations, said Britain's defence ministry.

"Such activity is placing further pressure on Russia’s already strained logistics and demonstrates the difficulties Russia is having re-organising its units in forward areas within Ukraine," it said in an assessment on Wednesday.

Russia is likely to continue to compensate for its reduced ground manoeuvre capability through mass artillery and missile strikes, the ministry added.

Russia has failed to capture any major city in its month-long invasion, while Ukrainian forces have made advances, recapturing territory from Russian troops on the outskirts of Kyiv, in the northeast and in the south.

One recaptured area on a road towards the village of Rusaniv was littered with burnt-out tanks and bits of Russian uniforms. Surrounding houses were destroyed.

Russia calls its assault a "special operation" to disarm and "denazify" Ukraine. The West says it launched an unprovoked invasion. The largest attack on a European nation since World War Two has killed or injured thousands, forced nearly 4 million to flee abroad and pummeled Russia's economy with sanctions.

Russian Deputy Defence Minister Alexander Fomin said the offer to scale back some military operations was a confidence building step for the ongoing negotiations with Ukrainian officials in Istanbul.

"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 told reporters.

Fomin made no mention of other areas that have seen heavy fighting, including around Mariupol in the southeast, Sumy and Kharkiv in the east and Kherson and Mykolaiv in the south.

"Ukrainians are not naive people," President Zelenskiy said late on Tuesday.

"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."

MAJOR OFFENSIVE POSSIBLE

Russia has started moving very small numbers of troops away from positions around Kyiv in a move that is more of a repositioning than a retreat or a withdrawal from the war, the Pentagon said on Tuesday.

"We all should be prepared to watch for a major offensive against other areas of Ukraine," spokesman John Kirby told a news briefing. "It does not mean that the threat to Kyiv is over."

Britain's Ministry of Defence earlier 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".

Reuters could not immediately verify the claims made by either side.

The Moscow-backed self-proclaimed Donetsk People's Republic in eastern Ukraine may consider joining Russia once it controls all of Ukraine's Donetsk region, its leader was quoted as saying. Kyiv has said any such move would have no legal basis.

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

Those who remain are suffering.

"We are eight people. We have two buckets of potatoes, one bucket of onions," said Irina, an engineer, in her apartment where windows had been blasted out.

In the southern city of Mykolaiv, a missile blasted a hole through the main administrative building. Authorities said at least 12 people were killed and 33 injured. read more

The Russian military accused Ukrainian forces in cities under attack of using ceasefires to restore their combat readiness and set up firing points in hospitals and schools, Interfax news agency said.

The leaders of Germany, the United States, France, Britain and Italy agreed in a phone call on Tuesday afternoon to keep pushing Russia for a ceasefire and for the withdrawal of its troops from Ukraine, a German government spokesman said.

Russian President Vladimir Putin and his French counterpart Emmanuel Macron discussed developments in the situation around Ukraine, including the latest round of Russia-Ukraine talks in Istanbul, in a phone call on Tuesday, the Kremlin said.

Russian Foreign Minister Sergei Lavrov and U.S. deputy national security adviser for economics, Daleep Singh, will both visit India to lobby New Delhi, which has called for a ceasefire but has refused to explicitly condemn Moscow.

PROPOSALS

Ukrainian negotiators said that under their 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 transatlantic NATO military alliance.

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

The fate of the southeastern 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.

Medinsky said Russia's delegation would study and present the proposals to president Putin.

To prepare a peace agreement, Medinsky later told the TASS news agency, "We still have a long way to go".