Moscow denies accusations of extrajudicial killings in the Ukrainian city of Izyum, while Kiev says a Russian missile has fallen near a nuclear power plant in the southern region of Mykolaiv.
The Kremlin has said reports of mass graves discovered in the formerly Russian-controlled city of Izyum in eastern Ukraine are fabrications.
"These are lies," Kremlin spokesperson Dmitry Peskov said on Monday, adding Moscow "will stand up for the truth in this story".
For Peskov it was "the same scenario as in Bucha", a town outside the capital Kiev where dozens of bodies in civilian clothing were found in the streets after Russian troops left in the early stages of the nearly seven-month-old conflict.
Moscow has refuted accusations of extrajudicial killings there.
Last week, some 450 graves were discovered in forests outside Izyum after the strategic city was recaptured from the Russians by Ukrainian troops.
Ukrainian forces swept through the Kharkiv region this month after bursting through the front line, sending thousands of Russian troops fleeing and abandoning their tanks and ammunition, and clawing back swathes of territory, including Izyum.
Ukrainian President Volodymyr Zelenskyy said investigators at the site in Izyum had found evidence of torture, including bodies with hands tied, and accused Russian troops of committing "war crimes".
Ukrainian officials said 99 percent of the exhumed bodies showed signs of violent death.
Nuclear plant strike
As the fighting continues, Ukraine's nuclear energy agency Energoatom accused Moscow's troops of an attack on the country's second largest nuclear plant in the south.
The accusations come after the Moscow-controlled Zaporizhzhia nuclear plant in Ukraine — Europe's largest atomic facility — faced frequent shelling in recent months, raising fears of a nuclear incident.
On Monday, "the Russian army carried out a missile attack on the industrial site" of the Pivdennoukrainsk nuclear power plant, Energoatom said in a post on the Telegram messaging service.
It added that a "powerful explosion" took place "just 300 metres" from the facility's reactors but they were operating as "normal".
The strike damaged more than a hundred windows of the power station's building, the nuclear agency said, posting photos of glass shattered around the broken windows.
The agency also released photographs of what it said was a two-metre-deep crater from where the missile landed.
"Fortunately, no one among the power plant staff was hurt," Energoatom said.
Reacting to the strike, Zelenskyy said on Telegram: "Russia endangers the whole world. We have to stop it before it's too late."