After the withdrawal of Russian troops from the city of Lyman in eastern Ukraine, the authorities say they found around 200 graves and one mass grave.
The exhumations have already begun, wrote the military governor of the Donetsk region, Pavlo Kyrylenko, in his channel on the Telegram news service.
He also published photos showing many small wooden crosses and emergency services in white protective suits.
The strategically important small town, temporarily occupied by Russian troops, was recaptured by the Ukrainians in early October.
According to initial findings, the dead could be both Ukrainian soldiers and civilians, it said.
How many bodies were in the mass grave has yet to be determined.
According to the police, several small children and entire families were among the people killed.
Ukrainian media had already reported on the discovery of dozens of graves in Lyman last Wednesday.
Some of the victims are said to have died from heavy shelling during the Russian conquest of the site in May.
Mass graves had already been found in various parts of Ukraine in the past after the withdrawal of Russian troops.
The bodies found in the Kiev suburb of Bucha in early April caused particular international outrage.
Hundreds of dead civilians had been found there - some with signs of torture and their hands tied.
Since then, Bucha has been considered a symbol for the most serious war crimes.
Zelenskyj sees further progress in Kiev's offensive
Meanwhile, President Zelenskyj sees further progress in the defense offensive of the Ukrainian armed forces.
In the past week, 776 square kilometers of land have been liberated;
29 settlements in total, including 6 in the Luhansk region, he said.
Villages in which the occupiers had recently held mock referendums on joining Russia are now again under Ukrainian control.
Since the beginning of the offensive, a total of 2,434 square kilometers and 96 settlements have come back under Ukrainian control.
The liberation of the territories continues, he said.
The Russian occupiers must be driven out everywhere and defeated as aggressors, Zelenskyj said.
This is the only way wars can be prevented in the future.
In a new reaction to Russia's annexation of Ukrainian territories, Zelenskyy now recognized the Pacific archipelago of the southern Kuril Islands as Japanese sovereignty.
In a video message, he called on the world community to also recognize Russian territory as Japanese.
The Soviet Union conquered the areas in the north of Japan as the victorious power in World War II.
Japan and Russia, the legal successors of the Soviet Union, still have no peace treaty because of the territorial dispute.
"Russia has no right to these areas," Zelenskyy said.
Zelenskyj defuses statements on "preemptive strikes"
At the same time, in a television interview, Zelenskyy softened his controversial statements about a "preventive strike" against Russia.
“You have to do pre-emptive kicks, not attacks.
We are not terrorists, we are not attacking any other territory," Zelenskyy said in a BBC interview in English on Friday in Kyiv.
Even after all the suffering of the war, Ukraine is still not ready "to kill people like the Russians do".