Russian Foreign Minister Sergei Lavrov, in remarks published early on Saturday, said lifting Western sanctions on Russia was part of the peace negotiations, which he said were "difficult" but continue daily by video link.

Ukraine president Volodymyr Zelenskiy told Polish journalists that chances were high that the talks, which have not been held in person for a month, would end because of Russia's "playbook on murdering people," the Interfax news agency said.

Ukraine accuses Russian troops of atrocities in areas near the capital, Kyiv, that they had occupied. Moscow denies the claims.

After failing to capture the capital in the nine-week assault that has turned cities to rubble, killed thousands and forced five million Ukrainians to flee abroad, Moscow is now focusing on the east and south.

Russian forces captured Ukraine's southern city of Kherson and mostly occupied the southeastern port city of Mariupol, where the United Nations is making efforts to evacuate civilians and fighters holed up in a large steel plant.

Lavrov told China's official Xinhua news agency that 1.02 million people had been evacuated to Russia from Ukraine since the invasion began on February 24. Ukraine says thousands have been taken to Russia against their will.

Reuters could not independently verify the claims of either side.

Lavrov said the evacuees included 120,000 foreigners and people from Russian-backed breakaway regions of Ukraine - the so-called Donetsk and Luhansk people's republics that Russia recognised as independent just before President Vladimir Putin announced the invasion.

Moscow calls the war a "special military operation" to disarm and "denazify" Ukraine, defend Russian-speaking people from persecution and prevent the United States from using the country to threaten Russia.

Ukraine dismisses Putin's claims of persecution and says it is fighting an unprovoked land grab to fully capture Donetsk and Luhansk, which form the Donbas region.

Britain and the United States have voiced support for Ukraine in the peace talks but say it is vital to keep arming Kyiv. On Thursday, US President Joe Biden asked Congress for $US33 billion ($A47 billion) in new aid, more than $US20 billion ($A28 billion) of it in weapons.

Ukraine acknowledges losing control of some eastern towns and villages but says Moscow's gains have come at a heavy cost to a force already worn down from its defeat near the capital.

"We have serious losses but the Russians' losses are much, much bigger," Ukrainian presidential adviser Oleksiy Arestovych said, without elaborating. "They have colossal losses."

Russia was pounding the entire Donetsk front line with rockets, artillery, mortar bombs and aircraft in part to stop Ukrainian troops from regrouping, Ukrainian officials said.

Ukraine's military said Russia was preparing for offensives in the areas of Lyman in Donetsk and Sievierodonetsk and Popasna in Luhansk. In the south, it said, Russia was "continuing to regroup, increase fire effectiveness and improve position".

Russia's defence ministry said its forces had struck Ukrainian weapons storage sites, platoon strongholds, artillery positions and drones. Russia said a diesel submarine in the Black Sea had struck military targets with Kalibr cruise missiles, the first report of such strikes from a submarine.

Russia said its high precision long-range missiles had destroyed the production facilities of a rocket plant in Kyiv.

Ukraine says that attack Thursday struck a residential building, injuring civilians and killing a producer with US-backed Radio Free Europe/Radio Liberty.

The body of the producer, Vira Hyrych, was found in the building's rubble, the broadcaster said.

Western officials said Russia had suffered fewer casualties after narrowing the scale of its invasion but numbers were still "quite high".

Australian Associated Press