As Russia retaliated with attacks on some of the recovered terrain, Ukraine forces said Monday that their lightning counteroffensive had gained more land in the last 24 hours.
Although Moscow signalled it was no closer to agreeing to a negotiated solution, the territorial movements were one of Russia's biggest U-turns since its forces were driven back from Kyiv in the first days of the nearly seven months of combat.
“Ukraine has turned the tide in its favour, but the current counter-offensive will not end the war,” US think tank Institute for the Study of War tweeted.
A day after Kyiv claimed that Russian airstrikes on the country's electrical infrastructure were to blame for power outages, Moscow announced air, missile, and artillery attacks on regained territories in the Kharkiv region on Monday.
As Ukraine claimed that its forces had retaken more than 20 additional settlements and that "Russian troops are quickly abandoning their posts and fleeing," it opened fire in retaliation.
The liberation of the important eastern city of Izyum, one of many triumphs claimed against the Russian army, was previously declared by Kyiv.
In addition to the enormous gains made in the east over the weekend, Ukraine said on Monday that its forces had recaptured 500 square kilometres (193 square miles) in the southern Kherson region.
Moscow conceded having lost territory, which experts saw as a serious blow to its war ambitions, but Kremlin spokesman Dmitry Peskov saw no prospects for negotiations.
“The special military operation continues and will continue until the objectives that were originally set are achieved,” he added, using Russia’s terminology for the internationally condemned war.
Eastern parts of Ukraine were hit with widespread electricity blackouts on Sunday evening, which President Volodymyr Zelensky said deliberately hit civilian infrastructure. He blamed “Russian terrorists”.
“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.”
‘Weapon, weapons, weapons’
Local Ukrainian authorities pointed to Russian strikes on their power infrastructure, but some districts reported later that power had been restored.In the 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”.
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.
The blackouts hit regions with an estimated combined population of nine million people — including territory controlled by Russia.
The Russian attacks also disrupted 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, had been shut off as a safety measure.
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”.
Around Balakliya, one of the first towns to be retaken by Ukrainian troops, AFP journalists saw evidence of fierce battles, with buildings destroyed or damaged and streets mostly deserted.
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.
Russia’s 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.