IZIUM, Ukraine (AP) — Ukrainian authorities were expected to begin recovering bodies 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.”

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.

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.

Associated Press journalists who visited the burial site Thursday counted hundreds of graves amid the trees, marked with simple wooden crosses — most of them numbered, up to 400 and beyond.

There was also at least one mass grave, with a marker saying it contained the bodies of 17 Ukrainian soldiers.

In his nightly televised address on Thursday, Zelenskyy said “more information — clear, verifiable information” about the burial site is expected Friday.

“The necessary procedural actions have already begun there,” he said. "We want the world to know what is really happening and what the Russian occupation has led to.”

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."

Before exhumation work could begin, investigators with metal detectors were scanning the site for any hidden explosives.

The mass grave of Ukrainian soldiers could contain more than the 17 bodies mentioned on its marker, said Oleg Kotenko, an official with the Ukrainian ministry tasked with reintegrating occupied territories.

“We haven’t counted them yet, but I think there are more than 25 or even 30,” he said, basing his estimate on video footage of the site that Russian soldiers posted on social media.

Izium resident Sergei Gorodko said that among the hundreds buried in individual graves were dozens of adults and children killed in a Russian airstrike on an apartment building.

He said he pulled some of them out of the rubble “with my own hands.”

Sergei Bolvinov, a senior investigator for Ukrainian police in the eastern Kharkiv region, told British TV broadcaster Sky News that a pit containing more than 440 bodies was discovered near Izium after Kyiv’s forces swept in. He described the grave as “one of the largest burial sites in any one liberated city.”

Some of the people buried in the pit were shot. Others died from artillery fire, mines or airstrikes. Many of the bodies have not been identified yet, Bolvinov said.

Russian forces left Izium and other parts of the Kharkiv region last week amid the stunningly rapid Ukrainian counteroffensive. On Wednesday, Zelenskyy made a rare trip outside the capital to watch the national flag being raised over Izium’s city hall.

Ukrainian Deputy Interior Minister Yevhen Enin said Thursday night that other evidence found after Kyiv’s sweeping advance into the Kharkiv region included multiple “torture chambers” where both Ukrainian citizens and foreigners were detained “in completely inhuman conditions.”

“We have already come across the exhumation of individual bodies, not only with traces of a violent death, but also of torture — cut off ears, etc. This is just the beginning,” Enin said in an interview with Ukraine’s Radio NV.

“All these traces of war crimes are now carefully documented by us. And we know from the experience of Bucha that the worst crimes can only be exposed over time,” Enin said, in a reference to a Kyiv suburb where the bodies of hundreds of civilians were discovered following the Russian army’s withdrawal from the area in March.

The $600 million in additional U.S. military aid announced Thursday will include more of the same types of ammunition and equipment that have helped the Ukrainian counteroffensive beat back Russian forces in large portions of the east and in the south.

The aid is “carefully calibrated to make the most difference on the battlefield and strengthen Ukraine’s hand at the negotiating table when the time is right,” Secretary of State Antony Blinken said in a statement. He praised the ”admirable grit and determination” of Ukrainians “defending their homeland and fighting for their future.”

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.

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Hanna Arhirova in Kyiv contributed reporting.