Ukrainian police say there are 445 new graves at the site, but some contain more than one body. It's unclear how all of them died. Many are said to be civilians, women and children among them.

They are trying to establish the cause of death of hundreds of people buried in a forest at the edge of the city, recently liberated by advancing Ukrainian forces.

Kharkiv regional prosecutor Olexander Ilyenkov says there is no doubt war crimes have been committed here.

"In the first grave, there is a civilian who has a rope over her neck. So we see the traces of torture," he told the BBC.

He said almost everyone died because of Russian soldiers.

"Some of them were killed, some were tortured, some were killed because of Russian Federation air and artillery strikes."

The burial ground - beside an existing cemetery - contains row after row of graves, marked by crude wooden crosses.

Names were written on a few, but most were marked only by a number. The burials here were carried out under the orders of the Russians when they were in control.

A senior advisor to Ukraine's President Volodymyr Zelensky told the BBC evidence of torture was found in some areas recently retaken by Ukrainian forces.

"We saw wildly frightened people who were kept without light, without food, without water, and without the right to justice," Mykhailo Podolyak said.

Kharkiv prosecutor Mr Ilyenkov said several similar burial sites had been found in areas recently retaken by Ukrainian forces.

US national security spokesman John Kirby said reports of the graves in Izyum were "horrifying" but "in keeping with the kind of depravity and the brutality with which Russian forces have been prosecuting this war against Ukraine".

"We're going to continue to actively support efforts to document war crimes and atrocities that Russian forces commit in Ukraine and to assist national and international efforts to identify and hold Russians accountable," he added.

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