Ukraine’s Ministry of Reintegration of Temporarily Occupied Territories reported the discovery Wednesday in a statement, providing further information on alleged Russian war crimes. The report comes roughly a month after Ukrainian officials jubilantly announced a counteroffensive had succeeded in pushing Russian forces out of northeast and southern regions of Ukraine.

Ukrainian forces have reclaimed more than 6,000 square kilometers (2,316 square miles) from Russia since early September. Ukrainians in these areas have suffered alleged human rights abuses under occupying Russian forces and many residents remain missing.

The ministry said Commissioner for Missing Persons Oleh Kotenko is at work in Ukraine’s recently reclaimed territories with local law enforcement agencies and search groups.

The roughly thousand bodies of military personnel and civilians exhumed includes children, and the ministry reported that 450 were found in a mass grave in Izium, a strategically important city in Ukraine’s northeastern Kharkiv region that was liberated during the counteroffensive.

“This is perhaps the most difficult segment of our search activity,” Kotenko said in the Reintegration Ministry’s statement.

Ukrainian forces have also made gains in the Kherson region in southern Ukraine, as well as in Luhansk and Donetsk in the country’s east. The ministry did not provide further details on where the other bodies were exhumed.

The United Nations Office of the High Commissioner for Human Rights (OHCHR) on Monday reported that a total of 6,374 Ukrainian civilians had been killed and 9,776 injured since Russia launched its February 24 invasion of the country. The office assigned most of the recorded civilian casualties to the use of explosive weapons, and noted the actual number is likely “considerably higher.”

The Kremlin has denied targeting civilians during the conflict.

The U.N. announced last month that it had dispatched human rights investigators to look into the reported mass grave in Izium. Human Rights Watch released a report last week finding evidence of Russian forces torturing detainees in Izium.

Yevhen Zhukov, head of the patrol department of Ukraine’s National Police, announced in early October that 180 bodies—including “whole families” with young children—had been found in a mass grave in Lyman, a recently reclaimed city in Ukraine’s Donetsk region.

As Russian troops pulled back earlier in the spring after failing to take Kyiv, reports emerged of hundreds of bodies found in the streets in nearby Bucha.

Newsweek reached out to the Russian Ministry of Defense for comment.