Ukraine accused the Russian military of attacking civilian infrastructure in response to a rapid weekend offensive by Ukrainian troops that forced Russia to abandon its main bastion in the Kharkiv region.
Ukrainian officials said the targets of the retaliatory attacks included water facilities and a thermal power station in Kharkiv, and that they caused widespread blackouts.
"No military facilities, the goal is to deprive people of light & heat," Ukrainian President Volodymyr Zelensky wrote on Twitter late on Sunday.
The US ambassador to Ukraine, Bridget Brink, also denounced the strikes.
"Russia's apparent response to Ukraine liberating cities and villages in the east: sending missiles to attempt to destroy critical civilian infrastructure," Brink tweeted.
Moscow denies its forces deliberately target civilians.
Zelensky has described Ukraine's offensive in the northeast as a potential breakthrough in the six-month-old war, and said the winter could see further territorial gains if Kyiv received more powerful weapons.
In the worst defeat for Moscow's forces since they were repelled from the outskirts of the capital Kyiv in March, thousands of Russian soldiers left behind ammunition and equipment as they fled the city of Izium, which they had used as a logistics hub.
Ukraine's chief commander, General Valeriy Zaluzhnyi, said the armed forces had regained control of more than 3,000 square km (1,158 square miles) since the start of this month.
"Kozacha (Lopan) is and will be Ukraine."Vyacheslav Zadorenko, district mayor
Near the Russian border, in the village of Kozacha north of Kharkiv, Ukrainian soldiers and local officials were greeted by residents with hugs and handshakes.
"Kozacha (Lopan) is and will be Ukraine," district mayor Vyacheslav Zadorenko said on a video he posted on Facebook. "No 'Russian World' whatsoever. See for yourselves where the 'Russian World' rags are lying around. Glory to Ukraine, glory to the Ukrainian Armed Forces."
Moscow's almost total silence on the defeat - or any explanation for what had taken place in northeastern Ukraine - provoked significant anger among some pro-war commentators and Russian nationalists on social media. Some called on Sunday for President Vladimir Putin to make immediate changes to ensure ultimate victory in the war. Read full story
Zelensky said late on Sunday that Russian attacks caused a total blackout in the Kharkiv and Donetsk regions, and partial blackouts in the Zaporizhzhia, Dnipropetrovsk and Sumy regions.
Mykhailo Podolyak, an adviser to Ukraine's president, said Kharkiv’s CHPP-5 electricity station - one of the largest in Ukraine - had been hit.
"A coward 'response' for the escape of its own army from the battlefield," he said on Twitter.
Kyrylo Tymoshenko, the deputy head of the president's office, posted an image on Telegram of electrical infrastructure on fire but added power had been restored in some regions.
Ukraine's gains are important politically for Zelensky as he seeks to keep Europe united behind Ukraine - supplying weapons and money - even as an energy crisis looms this winter following cuts in Russian gas supplies to European customers.
'Snowball rolling down a hill'
Defense Minister Oleksii Reznikov said Ukraine needed to secure retaken territory against a possible Russian counterattack on stretched Ukrainian supply lines. He told the Financial Times that Ukrainian forces could be encircled by fresh Russian troops if they advanced too far.
But he said the offensive had gone far better than expected, describing it as a "snowball rolling down a hill."
"It's a sign that Russia can be defeated," he said.
Kyiv-based military analyst Oleh Zhdanov said the gains could bring a further push into Luhansk region, whose capture Russia claimed at the beginning of July.
"If you look at the map, it is logical to assume that the offensive will develop in the direction of Svatove - Starobelsk, and Sievierodonetsk - Lysychansk," he said.
Leonid Pasechnik, the head of the Luhansk People's Republic, was quoted by Russian news agencies saying Ukrainian forces were attempting to penetrate that region.
"Ukrainian sabotage and reconnaissance groups have not stopped their attempts to infiltrate the territory of the republic for the purpose of provocation and intimidating our citizens," he said, adding there had been "no retreat from positions held by the republic."
Britain's defense ministry said on Sunday that fighting continued around Izium and the city of Kupiansk, the sole rail hub supplying Russia's front line across northeastern Ukraine, which has been retaken by Ukraine's forces.
Nuclear reactor shuts down
As the war entered its 200th day, Ukraine on Sunday shut down the last operating reactor at Europe's biggest nuclear power plant to guard against a catastrophe as fighting rages nearby.
Russia and Ukraine accuse each other of shelling around the Russian-held Zaporizhzhia plant, risking a release of radiation.
The International Atomic Energy Agency said a backup power line to the plant had been restored, providing the external electricity it needed to carry out the shutdown while defending against the risk of a meltdown.
French President Emmanuel Macron told Putin in a phone call on Sunday the plant's occupation by Russian troops is the reason why its security is compromised, the French presidency said. Putin blamed Ukrainian forces, according to a Kremlin statement.