Ukrainian authorities were expected to begin recovering bodies on Friday from a newly found mass burial site in a forest recaptured from Russian forces, a delicate task that President Volodymyr Zelenskyy said would help show the world "what the Russian occupation has led to."

Ukrainian authorities found a mass grave containing 440 bodies in a northeastern city in the town of Izium, which was recently liberated from Russian occupation, calling it proof of war crimes carried out by the invaders in territory they had occupied for months.

"Russia is leaving death behind it everywhere and must be held responsible," Zelenskyy said in a video address overnight.

"We want the world to know what the Russian occupation has caused," he said, without giving details on the number of bodies found or their cause of death.

"We need to have more clear and verified information tomorrow," he added.

"The necessary procedural actions have already begun there," he said.

Zelenskyy invoked the names of other Ukrainian cities where authorities said retreating Russian troops left behind mass graves of civilians and evidence of possible war crimes.

"Bucha, Mariupol, now, unfortunately, Izium," he said. "Russia leaves death everywhere. And it must be held accountable for it."

The U.N. Human Rights Office said on Friday that it plans to send investigators to the northeastern Ukrainian city of Izyum, where hundreds of graves were discovered following the withdrawal of Russian occupiers.

The find is shocking and the cause of death of each of the deceased must be investigated, a spokesperson for the U.N. agency in Geneva said.

The burial site, containing hundreds of graves, was discovered close to Izium after a rapid counteroffensive by Ukrainian forces liberated the northeastern city and other swaths of the Kharkiv region, breaking what was largely becoming a military stalemate in the nearly seven-month war.

The mass grave discovered in the former Russian front-line stronghold of Izium would be the biggest in Europe since the aftermath of the Balkan wars of the 1990s. Ukrainian forces retook Izium after thousands of Russian troops fled the area, abandoning weapons and ammunition.

"For months a rampant terror, violence, torture and mass murders were in the occupied territories," Zelenskyy adviser Mykhailo Podolyak tweeted in English, above a photo of a forest scattered with wooden crosses in fresh muddy ground.

"Anyone else wants to 'freeze the war' instead of sending tanks? We have no right to leave people alone with the Evil."

Russia did not immediately comment on the reports of the mass grave. In the past, it has denied its troops committing atrocities. Moscow calls its actions in Ukraine a "special military operation" to disarm its neighbor.

In Kupiansk, a northeastern railway junction city whose partial capture by Kyiv's forces on Saturday cut Russia's supply lines and led to the swift collapse of its front lines in the region, small units of Ukrainian troops were securing a nearly deserted ghost city.

'Mostly civilians'

Most of the people buried in a mass grave discovered in the eastern Ukrainian city of Izium are civilians, Ukraine's police chief said on Friday, based on a preliminary estimate.

Earlier, authorities said they had found a mass grave containing 450 bodies in Izium, a former Russian frontline stronghold, and said this was proof of war crimes carried out by the invaders. Russia has not publicly commented on the matter.

Asked if the mass grave contained mainly civilians or soldiers, police chief Ihor Klymenko told a news conference: "On a preliminary estimate, civilians. Although we have information that there are troops there, we haven't recovered a single one yet."

Meanwhile, to keep the offensive going, the Biden administration announced another $600 million package of military aid Thursday for Ukraine, including more of the weaponry that has helped its troops seize momentum.

The White House said it was the 21st time that the Defense Department has pulled weapons and other equipment off the shelves to deliver to Ukraine.

Ukraine is also getting military aid from the European Union. Making her third wartime visit this week to Kyiv, Ukraine's capital, EU chief Ursula von der Leyen said more aid is "absolutely vital and necessary."

Air raid sirens blared twice in Kyiv as von der Leyen met with Zelenskyy, a reminder that Russia has long-range weapons that can reach any location in Ukraine even though the capital has been spared attacks in recent weeks.

Russian forces suffered a stunning reversal this month after Ukrainian troops made a rapid armored thrust in the Kharkiv region in its northeast, forcing a rushed Russian withdrawal.

Zelenskyy on Wednesday made a surprise visit to Izium – until four days ago Russia's main bastion in the Kharkiv region – where he watched as the blue-and-yellow Ukrainian flag was raised in front of the charred city council building.