Ukrainian authorities found a mass grave containing 440 bodies in the northeastern town of Izium that was recaptured from Russian forces days ago, officials said, including some people killed by shelling and air strikes.
Thousands of Russian troops fled Izium last weekend after occupying the city and using it as a logistics hub in the Kharkiv region. They left behind large amounts of ammunition and equipment.
"Mass graves are being discovered in Izium after liberation from the (Russians)", with the largest burial site holding 440 unmarked graves, the Ukrainian defence ministry tweeted.
Serhiy Bolvinov, the chief police investigator for Kharkiv region, told Sky News: "Some died because of artillery fire ... some died because of air strikes."
Reuters could not immediately verify the Ukrainian reports and there was no immediate comment from Russia.
Ukrainian President Volodymyr Zelenskiy, who visited Izium on Wednesday, compared the discovery to alleged war crimes by Russian forces against civilians in Bucha, on the outskirts of the capital Kyiv, in the early stages of war.
"Russia is leaving death behind it everywhere and must be held responsible," Zelenskiy said in a video address late on Thursday.
Russia has denied targeting civilians or committing war crimes.
After a week of rapid Ukrainian gains in the northeast, Ukrainian officials said Russian forces were fortifying defences and it would be hard to maintain the pace of the advance.
Russian President Vladimir Putin has yet to comment publicly on the setback suffered by his forces this month. Ukrainian officials say 9,000 sq km (3,400 sq miles) have been retaken, territory about the size of the island of Cyprus.
Reuters video from the eastern town of Kupiansk, a key supply hub which Ukrainian forces recaptured last week, showed many buildings had been damaged or burned 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," said one man.
The speed of the advance has bolstered Ukrainian morale and given it some success to show its Western allies as it seeks more weapons to sustain its defence.
In Washington, U.S. President Joe Biden announced a new $600 million arms package to help Ukraine, including High Mobility Artillery Rocket Systems (HIMARS) and artillery rounds. The United States has sent about $15.1 billion dollars in security assistance to Kyiv since Russia's Feb 24. invasion.
Russians forces shelled Kharkiv city and other towns in the region on Thursday, Ukrainian officials said.
More than 90 missiles and artillery shells also targeted a region on the border with Russia, Dmytro Zhyvytsky, governor of Sumy region, said.
On the Russian side, Vyacheslav Gladkov, governor of Russia's Belgorod region, said the Ukrainian army shelled the town of Valuyki near the border.
"Anti-aircraft defences went into action but there is some destruction on the ground," he said on Telegram. This included a power substation put out of commission and private homes and vehicles set on fire.
Reuters was not able to verify battlefield reports.
CHINA'S CONCERNS ABOUT UKRAINE
Russia's 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.
Since Russia invaded Ukraine, China has trod a careful line, 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," Putin told Xi in Uzbekistan at their first meeting since the war began.
Xi did not mention Ukraine in his public remarks, nor was it mentioned in a Chinese account of the meeting.
Russian Foreign Minister Sergei Lavrov later told reporters the talks with China had been excellent.
The last time Putin and 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.
Pope Francis said it was morally legitimate for nations to supply weapons to Ukraine to help the country defend itself.
"This is a political decision which it can be moral, morally acceptable, if it is done under conditions of morality," Francis told reporters while travelling from a trip to Kazakhstan.
Francis also urged the Kyiv government to be open to eventual dialogue, even though it may "smell" because it would be difficult for the Ukrainian side.