Firefighters work at a site of the 5th thermal power plant damaged by a Russian missile strike in Kharkiv. Reuters

Throughout the weekend Ukraine claimed new military successes against Russia's army in the east and south, the latest coming Sunday night when President Volodymyr Zelensky declared Izyum "liberated".

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Russia 'regroups' troops in east as Ukraine advances

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Moscow conceded having lost territory, and experts have said the confirmed recapture by Ukraine of the city would represent a serious blow to Moscow's military ambitions in east Ukraine.

Sunday evening the eastern region experienced widespread electricity blackouts, which Zelensky said deliberately hit civilian infrastructure. He blamed "Russian terrorists".

Residents greet soldiers in a location given as Kozacha Lopan, Kharkiv Region, Ukraine, on Sunday. Reuters

"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.

"No military facilities," he added. "The goal is to deprive people of light and heat."

Local Ukrainian authorities pointed to Russian strikes on their power infrastructure. Some districts reported later that power had been restored.

In Kharkiv region, a Russian strike on a power station killed one employee, Governor Oleg Synegubov said. He added that power had been partially brought back.

Ukraine's foreign ministry spokesman Oleg Nikolenko said the Russian attacks were an "act of desperation following Russia's immense losses and retreat in eastern Ukraine".

Ukraine's national train service announced delays throughout the east including the country's second-largest city, Kharkiv.

The Russian strikes hit 15 locations on Sunday, from Kramatorsk in the east to Mykolaiv in the south and Dnipro in between, Ukraine's military said.

Earlier, the bombardment of Pokrovsk, a city in the eastern Donetsk region, left six residents dead, Mayor Rouslan Trebouchkine said.

The blackouts hit regions with an estimated combined population of nine million people -- including territory controlled by Russia.

They came as Ukrainian forces claimed to have recaptured dozens of towns and villages in eastern Ukraine.

The Russian attacks were also disrupting railway services. Ukraine's national train service announced delays throughout the east including the country's second-largest city, Kharkiv.

Ukraine had already lost all power from the Russian-controlled Zaporizhzhia nuclear plant.

The country's nuclear energy agency said the final reactor at the 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 warned that Ukrainian attacks near the station could have "catastrophic consequences".

'Weapons, weapons, weapons'

Sunday's address marked 200 days since the beginning of Russia's invasion.

President Volodymyr Zelensky speaks in Kyiv. File photo

The speed of Ukraine's fightback has apparently caught Russia'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 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".

Agence France-Presse