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, Ukrainian authorities said they had found a mass grave containing 440 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. Its forces fled Izium during a major Ukrainian counter-offensive that retook most of the northeastern Kharkiv region in the past week.

Asked if the Izium site 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 soldiers there too, we haven't recovered a single one yet."

The exhumations are continuing, he added.

Apart from those buried in the mass grave, Klymenko said that around 50 dead civilians had been found so far in the Kharkiv region in the past week. He did not elaborate.

Klymenko later told Ukrainian television that seven Sri Lankan students had been imprisoned and tortured in the town of Vovchansk near the Russian border. He said their fingernails had been pulled out with pliers. Reuters could not independently verify the report.