Mass grave containing hundreds of bodies discovered in Izium, Ukraine

Officials say there are 440 bodies at the site in the north-eastern town of Izyum, where thousands of Russian troops fled last weekend

A mass grave containing the bodies of Ukrainian soldiers and civilians was found in the forest near the recently recaptured city of Izium. AP

Ukrainian authorities have uncovered a mass grave containing hundreds of bodies in a town it recaptured from Russian forces days ago.

Officials say there are 440 bodies at the forest site in the north-eastern town of Izyum, where thousands of Russian troops fled last weekend after occupying the city for several months.

Some of the victims died as a result of shelling and air strikes, they said.

"Mass graves are being discovered in Izyum after liberation from the [Russians]", with the largest burial site holding 440 unmarked graves, the Ukrainian defence ministry tweeted.

Serhiy Bolvinov, chief police investigator for the Kharkiv region, told Sky News: "Some died because of artillery fire ... some died because of air strikes."

The reports could not be verified by reporters on the ground and there was no immediate comment from Russia.

Ukrainian President Volodymyr Zelenskyy, who visited Izyum on Wednesday, compared the discovery with the alleged war crimes by Russian forces against civilians in Bucha, on the outskirts of the capital Kyiv, in the early stages of the war.

"Russia is leaving death behind it everywhere and must be held responsible," he said in a video address late on Thursday.

Russia has denied targeting civilians or committing war crimes.

Ukraine said it has managed to take back 9,000 square kilometres of territory from the Russians since the start of the month. But it said it will be hard to maintain the pace of the advance because the Russian forces are now fortifying defences.

A video filmed in the eastern town of Kupiansk, a key supply centre that Ukrainian forces recaptured last week, showed many buildings had been damaged or burnt out.

"No electricity, no communications ... if there were communications we could at least talk to family. If only there hadn't been all this bombing with everyone in their basements," one man said.

A volunteer places a cross on a grave of one of 15 unidentified people killed by Russian troops, during a ceremony in the town of Bucha. Reuters

Russia's leader Vladimir Putin said on Thursday he understood China's leader Xi Jinping had concerns about the crisis in Ukraine, a surprise acknowledgement of friction with Beijing over the war.

China has trod a careful line since Russia launched the invasion, criticising western sanctions against Moscow but stopping short of endorsing or assisting in the military campaign.

"We understand your questions and concern about this. During today's meeting, we will of course explain our position," Mr Putin told Mr Xi in Uzbekistan at their first meeting since the war began.

Mr Xi did not mention Ukraine in his public remarks, nor was it mentioned in a Chinese account of the meeting.

Russian Foreign Minister Sergey Lavrov later told reporters the talks with China had been excellent.

The last time Mr Putin and Mr Xi met they signed a "no limits" friendship agreement between their countries. Three weeks later, Russia invaded Ukraine in what it called a "special military operation" to disarm its smaller neighbour.

Ukraine and the West accuse Russia of an unprovoked war of aggression.