KYIV, Ukraine — A day after routing Russian forces in a lightning advance that seized hundreds of square miles and a strategic town in the northeast, Ukraine claimed additional territory on Sunday in an offensive that has swiftly reshaped the battlefield in the nearly 200-day-old war and left Moscow reeling.
Ukraine’s rapid gains in the Kharkiv region have significantly weakened Russia’s hold on eastern Ukraine, which it has used as a stronghold to wage its war since February. Ukrainian officials said on Saturday that their troops had retaken the city of Izium, a strategically important railway hub southeast of Kharkiv that Russian forces seized in the spring after a bloody, weekslong battle.
On Sunday, President Volodymyr Zelensky claimed that Ukrainian forces had recaptured Chkalovske, a village in the Kharkiv region that lies roughly halfway between Izium and Kharkiv city. And the Ukrainian military command described continuing combat in cities in the Donetsk region, part of the stretch of mining towns and rolling fields known as Donbas, one of Russia’s main targets of the war.
Late in the day, in what Ukrainian officials condemned as a fit of pique over its losses, Moscow attacked infrastructure facilities in Kharkiv, leaving many civilians without power and water. Mr. Zelensky said there was a “total blackout” in the regions of Kharkiv and Donetsk.
“No military facilities,” he wrote on Twitter. “The goal is to deprive people of light and heat.”
The dizzying speed of Ukraine’s push in the past several days has smashed what had been a monthslong stalemate, buoyed a nation’s spirits and galvanized calls for even more Western military support. Gen. Valeriy Zaluzhnyi, the top commander of Ukraine’s armed forces, said that Ukraine had reclaimed more than 1,150 square miles of terrain since the start of September. A map released on Sunday by Russia’s defense ministry appeared to show that Ukrainian forces have driven Moscow’s troops from almost all of Kharkiv Province.
Russia’s retreat is the biggest embarrassment for President Vladimir V. Putin’s larger and better equipped forces since their attempt to seize Kyiv, the Ukrainian capital, was repelled at the start of the invasion. Amid heavy casualties, logistical problems and declining morale, Russia’s military performance has prompted discontent among pro-Kremlin bloggers and staunch Putin loyalists, creating new challenges for the Russian leader.
Here are other developments:
Ukraine’s gains have come with the help of increased intelligence sharing with the United States, American officials said.
Ukraine has begun turning off the last working reactor at the Zaporizhzhia nuclear power plant. The goal is to put it into a safe state as fighting swirls around the facility in southern Ukraine.
Russia’s front lines have collapsed in a crucial pocket of northeast Ukraine, ceding a wide area. Maps help show where Russian forces are retreating and how Ukraine’s offensive has reshaped what had been a grinding war of attrition.