LVIV, Ukraine/Kyiv outskirts - Ukraine and its Western allies on Wednesday (March 30) dismissed a Russian military pullback from near Kyiv as a ploy to refit troops after heavy losses, even as invading forces bombard cities elsewhere and press on with the obliteration of besieged Mariupol.

Nearly five weeks into an invasion in which it has failed to capture any major cities, Russia said it would curtail operations near Kyiv and the northern city of Chernihiv "to increase mutual trust" for peace talks.

But in an overnight address, Ukraine's President Volodymyr Zelenskiy made clear he took nothing Moscow said at face value.

"Ukrainians are not naive people," he said. "Ukrainians have already learned during these 34 days of invasion, and over the past eight years of the war in Donbas, that the only thing they can trust is a concrete result."

Ukrainian presidential advisor Oleksiy Arestovych said Moscow was shifting some forces from northern Ukraine to the east, where it was trying to encircle the main Ukrainian force there. Some Russians would stay behind near Kyiv to tie Ukrainian forces down, he said.

The past week has seen Ukrainian forces make gains, recapturing towns and villages on the outskirts of Kyiv, breaking the siege of the eastern city of Sumy and pushing back Russian forces in the southwest.

The Pentagon said Russia had started moving very small numbers of troops away from positions around Kyiv, describing the move as more of a repositioning than a withdrawal.

"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 defence ministry said Moscow was being forced to pull out troops from the vicinity of Kyiv to Russia and Belarus, to resupply and reorganise after taking heavy losses, adding that Russia was likely to compensate for its reduced ground manoeuvre capability through mass artillery and missile strikes.

Tacit admission 

Russia says it is carrying out a "special operation" to disarm and "denazify" its neighbour. Western countries say Moscow launched an unprovoked invasion, which included a full-scale assault on the capital that was repelled by fierce Ukrainian defence.

Moscow has said in recent days that its main focus is now on southeastern Ukraine, a region called the Donbas, where it is trying to capture more territory to turn over to separatists it has supported since 2014.

The area includes Mariupol, a port of 400,000 people which has been lain to waste after a month of Russian siege, and where the United Nations believes thousands of people may have died.

The British defence ministry, in an intelligence briefing, said the announcement that Moscow was now focusing on the southeast was "likely a tacit admission that it is struggling to sustain more than one significant axis of advance".

Around a quarter of Ukrainians have been driven from their homes by the biggest attack on a European country since World War Two. The United Nations said on Wednesday that the number who have fled the country had risen above four million. More than half of those refugees are children and the rest mostly women.

'Find roubles'

Western sanctions have isolated Russia from world trade to a degree never before visited on such a large economy. But Russia is still a major supplier of oil and gas to Europe, and Moscow has been trying to press that leverage.

Last week Moscow told Western buyers of its gas that they would now have to pay with roubles, a demand rejected by the G7 group of industrialised democracies.

On Wednesday, Germany, Russia's biggest gas customer, declared an "early warning" of a possible emergency if Russia cuts off supplies.

Economy Minister Robert Habeck urged consumers and companies to reduce consumption, saying "every kilowatt-hour counts". Supplies were safeguarded for the time being, but "nevertheless, we must increase precautionary measures to be prepared for an escalation on the part of Russia".

In a social media post, the speaker of Russia's parliament, Vyacheslav Volodin, said: "European politicians need to stop the talk, stop trying to find some justification about why they cannot pay in roubles."

"If you want gas, find roubles," he said.

He also proposed expanding the rouble payment policy to other exports - including oil, grain, fertiliser and metals.