Ukrainian forces have seized an expanding area of previously Russian-held territory in the east in a "very sharp and rapid" advance, a Russian-installed regional official said today.
The breakthrough may mark a turning point in the war.
After keeping silent for a day, Russia effectively acknowledged a section of its frontline had crumbled south east of Ukraine's second-largest city Kharkiv.
"The enemy is being delayed as much as possible, but several settlements have already come under the control of Ukrainian armed formations," Vitaly Ganchev, head of the Russian-backed administration in the Kharkiv region, said on state television.
Ukrainian President Volodymyr Zelensky later said Kyiv's forces had liberated more than 30 settlements in the Kharkiv region so far and that fighting continued in the eastern Donbas region and the south.
"Our army, intelligence units and the security services are carrying out active engagements in several operational areas. They are doing so successfully," he said in a video address.
Earlier today, the head of Ukraine's nuclear energy agency told AFP told Russian forces controlling Ukraine's Zaporizhzhia nuclear power plant have killed two staff at the facility and detained and abused dozens of others.
The plant - the largest in Europe - was captured by Russian troops in March.
An uptick in fighting around it in recent weeks has raised fears of a nuclear disaster with both Moscow and Kyiv blaming the other for the escalation.
"A regime of harassment of personnel was gradually established," following the Russian takeover, Petro Kotin said.
"Two people were beaten to death. We do not know where about ten people are now, they were taken (by the Russians) and after that we have no information about their whereabouts," Mr Kotin said, adding about 200 people had been detained.
He described the current situation at the plant as "very difficult," citing "torture" of staff and "beatings of personnel.
"The Russians look for pro-Ukrainian people and persecute them. People are psychologically broken," he said in an interview with AFP reporters in his office in Kyiv.
Earlier today, shelling near the Zaporizhzhia plant caused a blackout, compromising its safe operation, the International Atomic Energy Agency (IAEA) said today.
"Shelling has caused a complete blackout in Energodar (town) and compromised the safe operation of the nearby Zaporizhzhia (nuclear power plant)," IAEA director general Rafael Grossi said in a tweet, calling it a "dramatic development".
"This is completely unacceptable. It cannot stand," he said in a statement, urgently calling for "the immediate cessation of all shelling in the entire area".
"Only this will ensure the security of operating staff and allow the durable restoration of power to Energodar and to the power plant," Europe's largest nuclear power facility, he added.
Mr Grossi said he learned of the shelling today from IAEA staff on the site, evoking the "serious situation that developed last night".
Kyiv on Wednesday called for an international mission to be set up in the Zaporizhzhia nuclear power plant and called for the population to evacuate the area amid fears of a nuclear disaster.
The Zaporizhzhia power plant was occupied by Russian troops in March and has been shelled in recent weeks, with Ukraine and Russia blaming each other for the attacks, raising fears of a nuclear disaster.
