Thousands of Russian troops fled Izium last weekend after occupying the city and using it as a logistics hub in the Ukraine's eastern Kharkiv region.
They left behind large amounts of ammunition and equipment.
"I can say it is one of the largest burial sites in a big town in liberated (areas) ... 440 bodies were buried in one place," 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 claim and there was no immediate public comment from Russia.
President Volodymyr Zelenskiy, who paid a surprise visit to Izium on Wednesday to greet Ukrainian troops, put the blame squarely on Russia and likened the discovery to what happened in Bucha, on the outskirts of the capital Kyiv in the early stages of the late February invasion.
Ukraine and its Western allies have accused Russia of perpetrating war crimes there, which Moscow has denied.
"Russia is leaving death behind it everywhere and must be held responsible," Zelenskiy said in a video address late on Thursday.
After a week of rapid Ukrainian gains in the northeast, officials said Russian forces were fortifying defences.
Russian President Vladimir Putin has yet to comment publicly on the setback suffered by his forces, with Ukrainian officials saying 9000 sq km has been retaken.
The speed of the advance has lifted hopes of further gains before the northern winter sets in.
But Serhiy Gaidai, governor of Ukraine's eastern Luhansk region, said it would still be a tough fight to wrest back control of the region from Russia, which recognises it as an independent state controlled by separatists.
Throughout Thursday, Russian forces shelled towns in the Kharkiv, Kupiansk and Izium districts of the Kharkiv region, governor Oleh Synehubov said on Telegram.
More than 90 missiles and artillery shells targeted a region on the border with Russia until late on Thursday, Dmytro Zhyvytsky, governor of Sumy region, said.
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.
Reuters was not able to verify battlefield reports.
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 walked a careful line - criticising Western sanctions against Moscow but stopping short of endorsing or assisting in the military campaign.
"We highly value the balanced position of our Chinese friends when it comes to the Ukraine crisis," Putin told Xi in Uzbekistan at their first meeting since the war began.
"We understand your questions and concern about this. During today's meeting, we will of course explain our position."
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.
Meanwhile, 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 said in an airborne news conference 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.
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