Ukrainian forces said on Monday their Blitz counteroffensive had reclaimed more ground over the past 24 hours as Russia responded with strikes on some of the retaken areas.

The territorial shifts were one of Russia’s biggest reversals since its forces withdrew from Kyiv in the first days of nearly seven months of fighting, but Moscow signaled it was no closer to a negotiated peace.

“Ukraine has turned the tide in her favor, but the current counteroffensive will not end the war,” the US think tank Institute for the Study of War tweeted.

Moscow announced air, missile and artillery strikes on reclaimed areas in the Kharkiv region on Monday, a day after Kyiv said Russian attacks on power infrastructure had caused blackouts.

The retaliatory fire came as Ukraine said forces had retaken more than 20 other settlements and claimed “Russian troops are hastily abandoning their positions and fleeing”.

Kyiv had already announced the retaking of the strategic city of Izyum in the east of the country, one of a series of victories claimed against the Russian army.

Ukraine said Monday its forces had recaptured 500 square kilometers (193 sq mi) in the southern Kherson region, in addition to huge gains in the east over the weekend.

Moscow acknowledged losing territory, which experts saw as a major blow to its war ambitions, but Kremlin spokesman Dmitry Peskov saw no prospects for negotiations.

“The military special operation goes on and will continue until the initially set goals are achieved,” he added, using Russian terminology for the internationally condemned war.

Eastern parts of Ukraine were hit by widespread power outages Sunday night, which President Volodymyr Zelenskyy said were deliberately hitting civilian infrastructure. He blamed “Russian terrorists”.

“A total blackout in Kharkiv and Donetsk regions, a partial one in Zaporizhia, Dnipropetrovsk and Sumy regions,” Zelenskyy 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, but some districts later reported that power had been restored.

In the Kharkiv region, a Russian strike at a power plant killed an employee, Governor Oleg Synegubov said. He added that power has been partially restored.

Ukraine’s Foreign Ministry spokesman Oleg Nikolenko said the Russian attacks were an “act of desperation following the immense losses and Russia’s withdrawal to eastern Ukraine”.

Russian attacks hit 15 locations on Sunday, from Kramatorsk in the east to Mykolayiv in the south and Dnipro in between, the Ukrainian military said.

The blackouts hit regions with an estimated total population of nine million people – including areas controlled by Russia.

The Russian attacks also disrupted rail traffic. Ukraine’s national train service announced delays across the east, including the country’s second-largest city, Kharkiv.

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

The country’s nuclear energy agency said the last reactor at the power plant, Europe’s largest nuclear power plant, had been shut down for safety reasons.

The speed of the Ukrainian counterattack has apparently taken the Russian military by surprise, returning parts of territory that Moscow had controlled for months to Kiev.

Images posted by the Ukrainian military showed boxes of ammunition and military equipment scattered across the area deserted by Russian forces.

In his evening address on Sunday, Zelenskyy praised the soldiers who “liberated hundreds of our towns and villages … and most recently Balakliya, Izyum and Kupiansk”.

Around Balakliya, one of the first towns to be recaptured by Ukrainian troops, AFP journalists saw evidence of heavy fighting with buildings destroyed or damaged and roads mostly deserted.

The country’s foreign minister used the momentum to appeal to Western allies to stockpile more sophisticated weapons.

“Guns, guns, guns have been on our agenda since spring. I am grateful to the partners who answered our call: Ukraine’s successes on the battlefield are our common ones,” said Dmytro Kuleba.

Russia’s military surprisingly announced on Saturday that it would “regroup” its forces from Kharkiv to the southern Donetsk region to concentrate its military efforts there.