Ukraine on Sunday accused Russia of causing sweeping power cuts in the east of the country, as Kyiv announced fresh gains in its counter-offensive, including the recapture of the key city of Izyum.
Ukraine's nuclear energy agency had already said the final reactor at the Russian-controlled Zaporizhzhia plant, Europe's largest nuclear power station and a focal point of the conflict, had been shut off as a safety measure.
Russian President Vladimir Putin meanwhile warned that Ukrainian attacks near the station could have "catastrophic consequences".
Later Sunday, large areas of eastern Ukraine were plunged into power cuts, which Ukrainian officials blamed on Russia.
Ukraine's President Volodymyr Zelensky accused Moscow of having deliberately hit civilian infrastructure.
"A total blackout in the Kharkiv and Donetsk regions, a partial one in the Zaporizhzhia, Dnipropetrovsk and Sumy regions," Zelensky said in a statement on social media, blaming "Russian terrorists".
"No military facilities," he added. "The goal is to deprive people of light and heat."
Some districts reported that power had later been restored
The blackouts came as Ukrainian forces claimed to have recaptured dozens of towns and villages in eastern Ukraine.
The electricity cuts hit regions with an estimated combined population of nine million people -- including territory controlled by Russia.
The Russian attacks were also disrupting railway services, with the national train service announcing delays throughout the east including the country's second largest city, Kharkiv.
'Weapons, weapons, weapons'
The speed of Ukraine's fightback against Russia's invasion has apparently caught Moscow's military off-guard, bringing swathes of territory Moscow had controlled for months back into Kyiv's fold.
Images posted by the Ukrainian military showed crates of munitions and military hardware scattered across territory abandoned by the Russian forces.
In his evening address Sunday Zelensky praised the soldiers who had "liberated hundreds of our cities and villages... and most recently Balakliya, Izyum and Kupiansk," naming three important hubs recently captured by Kyiv's army.
Around Balakliya, one of the first towns to be recaptured by Ukrainian troops, AFP journalists saw evidence of fierce battles, with buildings destroyed or damaged and streets mainly deserted.
Sunday's address marked 200 days since the beginning of Russia's invasion.
Earlier Sunday, the head of the Ukrainian military announced that as much as 3,000 square kilometres (1,158 square miles) had been wrested from Russia since the offensive began at the beginning of this month.
The country's foreign minister used the momentum to appeal to Western allies for more stockpiles of sophisticated weapons.
"Weapons, weapons, weapons have been on our agenda since spring. I am grateful to partners who have answered our call: Ukraine's battlefield successes are our shared ones," Dmytro Kuleba said.
"Prompt supplies bring victory and peace closer," his statement on social media read.
Russia 'regrouping'
The reaction in Moscow to the Ukrainian gains so far has been muted, but on Sunday a military map presented by the Russian defence ministry showed that its forces had made a major withdrawal from the Kharkiv region.
Russia controlled just a sliver of territory in the region's east, behind the Oskil river, the revised map showed.
The Russian military made the surprise announcement Saturday that it was "regrouping" its forces from Kharkiv to the Donetsk region just south to focus its military efforts there. That came shortly after Moscow said it was actually sending reinforcements towards Kharkiv.
The fresh fighting in and around the Zaporizhzhia nuclear power plant has sharpened fears of another nuclear incident comparable to the Chernobyl disaster in northern Ukraine in 1986.
Putin, in a telephone call with French President Emmanuel Macron, once again accused Ukraine of repeated attacks on the Zaporizhzhia nuclear plant held by Russian troops.
The Russian leader drew attention to "regular Ukrainian attacks" which could have catastrophic consequences," the Kremlin said in a statement.
Each side has repeatedly blamed the other for shelling in the area.
Macron's office, in its readout of the conversation, said the French president had told Putin that "the Russian occupation was the reason for the risks" facing the plant.
Ukraine's state nuclear agency on Sunday said that the sixth and final reactor at the plant had been shut off.
Energoatom said the sixth reactor had been generating energy for the plant itself for three days and that the decision to halt its operations came when external power had been restored to the facility.
It cautioned again, however, that in its view the only way to ensure the safety of the facility would be to create a demilitarised zone around it.
In Balakliya, once of the towns recently recaptured by Ukrainian troops, 52-year-old Iryna Stepanenko, was outside cycling for the first time in months.
She had hidden in her basement for three months, she said from the town that was home to some 27,000 people lived before the invasion.
But while she was relieved that Kyiv's forces were back, she was still worried about the future.
"I'm worried the Russians could return. I'm worried the shelling could start again."
Despite the reported Ukrainian gains, Russian forces have continued bombardments across the frontline, and in the Donetsk region, officials said shelling killed 10 people and wounded another 19.