LVIV (REUTERS) - Ukrainian troops are recapturing towns east of Kyiv and Russian forces who had been trying to seize the capital are falling back on overextended supply lines, Britain said on Friday (March 25), one of the strongest indications yet of a shift in momentum in the war.

The mayor of a suburb east of Kyiv said Ukrainian troops had recaptured a nearby village and thousands of civilians were now leaving the area, answering a call from the authorities to get out of the way of the counter-attack.

A month into their assault, Russian troops have failed to capture any major Ukrainian city. An offensive Western countries believe was aimed at swiftly toppling President Volodymyr Zelensky's government was halted at the gates of Kyiv.

Russian troops have instead been bombarding, encircling and besieging Ukrainian cities, laying waste to residential areas and driving around a quarter of the country's 44 million people from their homes.

Russia’s defence ministry said on Friday that the first phase of its military operation in Ukraine was mostly complete and that it would focus on completely “liberating” eastern Ukraine’s Donbass region.

The announcement appeared to indicate that Russia may be switching to more limited goals after running into fierce Ukrainian resistance.

Russian news agencies quoted the defence ministry as saying that Russian-backed separatists now controlled 93 per cent of Ukraine’s Luhansk region and 54 per cent of the Donetsk region – the two areas that jointly make up the Donbass.

The ministry said it did not rule out storming Ukrainian cities that had been blockaded and that Russia would react immediately to any attempt to close the airspace over Ukraine - something Kyiv has asked Nato to do, but Nato has resisted.

Across the border in Poland, US President Joe Biden landed in the south-eastern city of Rzeszow on Friday – around 100km away from Ukraine.

Mr Biden is due to meet US soldiers stationed in the area and non-governmental organisations dealing with a refugee crisis which has seen 3.6 million Ukrainians flee abroad.

Battlelines near Kyiv have been frozen for weeks with Russian armoured columns threatening the capital from the northwest and the east. But in an intelligence update on Friday, Britain described a Ukrainian counter-offensive that had pushed Russians far back in the east.

"Ukrainian counter-attacks, and Russian forces falling back on overextended supply lines, has allowed Ukraine to reoccupy towns and defensive positions up to 35km east of Kyiv," the update said.

Volodymyr Borysenko, mayor of Boryspol, an eastern suburb where Kyiv's main airport is located, said 20,000 civilians had left the area, answering a call to clear out so Ukrainian troops could push the Russians further back.

Ukrainian forces had recaptured a village from Russian troops the previous day between Boryspol and Brovary, and would have pushed on further but had halted to avoid putting civilians in danger, he said.

On the other main front outside Kyiv, to the capital's northwest, Ukrainian forces have been trying to encircle Russian troops in the adjacent suburbs of Irpin, Bucha and Hostomel, reduced to ruins by heavy fighting over the past few weeks.

In Bucha, 25km northwest of Kyiv, a group of Ukrainian troops armed with anti-tank missiles were digging deep trenches to defend their frontline positions.

One of them, Andriy, told Reuters he had left his wife and children hiding in their house in an area that was later taken over by Russian forces.

"I told my wife to grab the children and to hide in the basement, and I went to the drafting station and joined my unit straight away. And the next day from the army base we moved to the frontline," he said.

"My wife and children were under occupation for two weeks, but then they managed to escape through a humanitarian corridor."

The Ukrainian military said its troops were repelling Russian forces trying to fight their way into Kyiv. Troops were also still holding on to the city of Chernihiv, northeast of Kyiv, hindering a Russian advance in the direction of the capital.

Chernihiv's governor said the city was effectively surrounded by Russian forces.

In Kharkiv, Ukraine's second largest city, which has sustained punishing bombardment since the war's early days, police said four people had been killed by the shelling of a centre for distributing humanitarian aid. Russia denies targeting civilians.

Moscow calls its actions in Ukraine a "special military operation" to disarm its neighbour. Kyiv and its Western allies call it an unprovoked war of aggression and say Russia's true aim was to overthrow the government of what President Vladimir Putin regards as an illegitimate state.

Unable to capture cities, Russia has resorted to pounding them with artillery and air strikes. The eastern port city of Mariupol has been under siege since the war's early days. Tens of thousands of people are still believed to be trapped inside with no access to food, medicine, power or heat.

In a part of the city now captured by the Russians, one woman waiting in line to receive food supplies told Reuters her diabetic husband had slipped into a coma and died. He was buried in a flowerbed.

"We are planning on leaving but it's very difficult at the moment," the woman, who gave her name as Alexandra, said. "I can't leave my husband in a flowerbed ... And then we have nowhere to go."

Anti-ship missiles

After attending an emergency summit of Nato and the G7 in Brussels on Thursday, Biden is visiting Poland, which has taken in more than half of Ukraine's refugees.

The West has ruled out intervening on the ground or answering Ukraine's plea for a no-fly zone, but has supported Kyiv with hundreds of anti-tank and anti-aircraft weapons that have blasted Russian armoured columns and prevented Moscow from taking control of the air.

A senior US administration official said Washington and its allies were also working on providing anti-ship weapons to protect Ukraine's coast. Ukrainian forces claimed on Thursday to have blown up a Russian landing ship in a Russian-occupied port.

US officials told Reuters that Russia is suffering failure rates as high as 60 per cent for some of its precision-guided missiles.

With stocks of precision-guided munitions running low, Russian forces were more likely to rely on unguided bombs and artillery, Under Secretary of Defence for Policy Colin Kahl said.