The announcement in Moscow appeared to indicate that Russia may be switching to more limited goals after running into fierce Ukrainian resistance in the first month of the war.

The defence ministry said 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 Donbass.

"The main objectives of the first stage of the operation have generally been accomplished," Sergei Rudskoi, head of the Russian General Staff's Main Operational Directorate, said.

"The combat potential of the Armed Forces of Ukraine has been considerably reduced, which... makes it possible to focus our core efforts on achieving the main goal, the liberation of Donbass."

Russia sent tens of thousands of troops into Ukraine on February 24 in what it called a "special operation" to weaken its neighbour's military capabilities and root out people it called dangerous nationalists.

Rudskoi said 1351 Russian soldiers had died in the operation and 3825 had been injured.

Russia's military had not given an official casualty figure since March 2, when it said 498 soldiers had been killed.

Ukraine's military has said about 15,000 Russian soldiers have been killed in combat.

Russia's General Staff said its military operation in Ukraine would continue until Russian forces had completed the tasks that had been set, without saying what those tasks were.

Russia's military had considered two options for its operation in Ukraine, one confined to the Donbass and the other on the whole territory of Ukraine, before opting for the latter, Rudskoi said.

The United Kingdom said on Friday that Ukrainian troops were recapturing towns east of Kyiv and Russian forces who had been trying to seize the capital were falling back on overextended supply lines, an indication of a shift in momentum in the fighting.

The UK has provided Ukraine with weapons and military training.

Battlelines near Kyiv have been frozen for weeks with two main Russian armoured columns stuck northwest and east of the capital.

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 counter-attack.

Ukrainian forces recaptured a nearby village the previous day and would have pushed on but 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 suburbs of Irpin, Bucha and Hostomel, reduced to ruins by heavy fighting over the past few weeks.

In the first big sign that foreign sanctions on Russia were affecting investment from China, sources told Reuters state-run Sinopec Group, Asia's biggest oil refiner, had suspended talks on a petrochemical investment and a venture to market Russian gas.

US President Joe Biden was visiting Poland for a first-hand look at the refugee crisis on Friday.

Poland has taken in more than half of the 3.7 million Ukrainians who have fled abroad.

Mariupol's city council for the first time gave an estimated death toll for the bombing of the main theatre on March 16, saying witnesses now said 300 people had been killed among many hundreds sheltering in the basement.

Russia denies blame.

The United Nations said it was looking into reports of mass graves inside Mariupol, including one with at least 200 corpses.

Australian Associated Press