Ukrainian authorities have begun unearthing bodies from a mass burial site in a forest recaptured from Russian forces, a find that President Volodymyr Zelensky said would help show the world “what the Russian occupation has led to”.

The site, which police said contained 445 graves, was discovered close to Izium after a rapid counteroffensive by Ukrainian forces retook the north-eastern city and much of the Kharkiv region, breaking what had largely become a stalemate in the nearly seven-month war.

To bolster the offensive, the Biden administration announced another 600 million dollar package of military aid on Thursday for Ukraine, including more of the weaponry that has helped its troops seize the momentum.

Associated Press journalists who visited the burial site on Thursday saw graves amid the pine trees, marked with simple wooden crosses. Most were numbered – and the count went past 400.

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Ukrainian President Volodymyr Zelensky (Efrem Lukatsky/AP)

It was not clear who was buried in many of the plots or how all of them died, though witnesses and a Ukrainian investigator said some were shot and others were killed by artillery fire, mines or air strikes.

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

In his nightly televised address on Thursday, Mr Zelensky said “more information – clear, verifiable information” about the burial site was expected soon.

“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.”

Mr Zelensky invoked the names of other Ukrainian cities where authorities said retreating Russian troops left behind mass graves of civilians.

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A Ukrainian serviceman sits in a tank, in the recently retaken area of Izium (Evgeniy Maloletka/AP)

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

The marking of individual graves with wooden crosses differed from some other burial sites discovered earlier in the war and seen by AP reporters – including some around Kyiv that are being investigated as sites of possible war crimes. Bodies found outside the capital in the town of Bucha and elsewhere after Russian forces withdrew had been dumped together and buried without markers.

Izium resident Sergei Gorodko said that among the hundreds buried in individual graves were dozens of adults and children killed in a Russian air strike 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, told Sky News that some of the people buried were shot, while others died from artillery fire, mines or air strikes.

Unidentified graves of civilians and Ukrainian soldiers in a cemetery in Izium
Unidentified graves of civilians and Ukrainian soldiers in a cemetery in Izium (Evgeniy Maloletka/AP)

The mass grave 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.

Mr Kotenko also said that individual graves marked with crosses contained civilians who died. He said he expected the bodies would be exhumed for DNA testing.

Before exhumation work could start, investigators with metal detectors scanned the site for any hidden explosives. Soldiers strung red and white plastic tape between the trees to mark off parts of the site. A few graves had wreaths of flowers hanging from the crosses, and some bore people’s names.

Izium was a key supply hub for Russian forces until they withdrew in recent days.

Izium city councillor Maksym Strelnikov told reporters in an online briefing from an undisclosed location this week that hundreds of people had died during the fighting and after Russia seized the town in March. Many died from shelling and could not get a proper burial, he said.

His claims could not be immediately verified, but similar scenes have played out in other cities captured by Russian forces, including Mariupol.

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”.