STORY: Ukraine extended its hold on recently recaptured territory on Monday as troops marched farther east into areas abandoned by Russia, paving the way for a potential assault on occupation forces in the Donbas region.
President Volodymyr Zelenskiy vowed Ukraine would keep fighting to regain its territory.
"We are stabilizing the situation, holding our positions. Firmly. So strong that the occupiers are really panicking. We have warned that the Russian soldiers in Ukraine have only two options - to flee from our land or surrender."
In a dramatic counter-offensive that has changed the tide of war, Ukrainian forces swept through the Kharkiv region this month, sending thousands of Russian troops fleeing and abandoning their tanks and ammunition.
The Ukrainian advance has slowed in recent days, but Zelenskiy said their forces were consolidating and preparing for further offensives.
Ukraine accused Russian forces on Monday of shelling near the Pivdennoukrainsk nuclear power plant in the south.
Zelenskiy released video footage of the explosion.
Ukraine's state nuclear company said buildings were damaged, but its reactors were unscathed. The attack also damaged a nearby hydroelectric power plant and transmission lines.
Since its forces were driven out of Kharkiv, Russia has repeatedly fired at power plants, water infrastructure and other civilian targets in what Ukraine says is retaliation for defeats on the ground.
Moscow, which says it is engaging in a special military operation, denies deliberately targeting civilians.
In the newly recaptured city of Izium, forensic experts carried out the grim task of exhuming bodies from a mass grave -- most of them, civilians.
Serhiy Bolvinov, the head of investigative police in the Kharkiv region, said some bodies showed signs of a violent death.
"As we can see, dead bodies of civilians and servicemen were buried without coffins. Because of that the shape of the dead bodies is not good. There are decomposed parts. During preliminary examination, it is very hard to establish the entire list of damages done to the bodies."
Zelenskiy has said some 450 bodies are believed to have been buried at the site in a forest on the outskirts of town.
The Kremlin on Monday rejected allegations that Russian forces had committed war crimes in Ukraine's Kharkiv province, saying it was a "lie."