Meanwhile, the Ukrainian presidency says it has agreed to hold negotiations with Russia at a location on the border between Ukraine and Belarus.

President Volodymyr Zelensky said that in a conversation with Belarusian leader Alexander Lukashenko, the Ukrainian side had agreed to meet the Russians without preconditions on the Ukrainian-Belarusian border, near the Pripyat River.

Mr Zelensky said Mr Lukashenko had taken responsibility for ensuring that all planes, helicopters and missiles stationed on Belarusian territory remained on the ground during the Ukrainian delegation's travel, talks and return.

Neither Russia nor Belarus have issued any statements about potential talks.

Russia's defence ministry has now acknowledged that its military personnel have been killed and wounded in Ukraine, though it claims the losses are much less than those of Ukraine.

The UN said that by Saturday, at least 134 Ukrainian military had been killed. Ukraine, meanwhile, says there have been 4,300 casualties among Russian servicemen, but even Ukrainian officials say those figures are not clear.

Ukraine's human rights commissioner says 210 civilians have been killed.

Among the dead are children - one when a children's hospital was shelled in Kyiv and one in a Russian attack on a kindergarten that claimed six lives near the northern border at Okhtyrka.

Nine civilians have been killed in Kyiv, while 10 ethnic Greek Ukrainians died in air strikes on villages in the south-east, Greek officials say.

So far, the UN has recorded 64 confirmed civilian deaths and many more wounded, and it expects the true number to be much higher.

In other developments, as the Russian invasion entered its fourth day: