Ukrainian troops on Sunday successfully pressed their swift counteroffensive in the northeastern part of the country, signaling what is widely seen as a potential turning point in the war which marked its 200th day on Sunday. This is so even as Russia struck back at Ukraine’s infrastructure Sunday night, causing widespread blackouts, with the Kharkiv and Donetsk regions among those without power, officials said.

But Kyiv’s action to reclaim Russia-occupied areas in the Kharkiv region forced Moscow to withdraw its troops to prevent them from being surrounded, leaving behind significant numbers of weapons and munitions. The Telegraph newspaper reported that panicked Russian soldiers have abandoned their tanks, weapons and even their uniforms, switching for civilian clothes, amid a near-total collapse of Russia’s front in the northeastern swath of the country.

President Zelensky mocked the Russians in a video address Saturday night, saying “the Russian army in these days is demonstrating the best that it can do — showing its back.” He posted a video of Ukrainian soldiers hoisting the national flag over Chkalovske, another town reclaimed in the counteroffensive.

Yuriy Kochevenko, of the 95th brigade of the Armed Forces of Ukraine, tweeted a video from what appeared to be the city center of Izyum. The city was considered an important command and supply hub for Russia’s northern front.

“Everything around is destroyed, but we will restore everything. Izyum was, is, and will be Ukraine,” Kochevenko said in his video, showing the empty central square and destroyed buildings.

Ukraine’s military chief, General Valerii Zaluzhnyy, said its forces had recaptured about 1,160 square miles since the counteroffensive began in early September. He said Ukrainian troops are only about 30 miles from the Russian border.

One battalion shared a video of Ukrainian forces in front of a municipal building in Hoptivka, a village just over a mile from the border and about 12 miles north of Kharkiv.

Kharkiv Governor Oleh Syniehubov said Ukrainian troops have reclaimed control of more than 40 settlements in the region.

Widespread power outages were reported Sunday night by Ukrainian media, with the Kharkiv and Donetsk regions completely blacked out, while Dnipropetrovsk, Zaporizhzhia and Sumy partially lost power, Zelensky said.

“Russian terrorists remain terrorists and attack critical infrastructure. No military facilities, only the goal of leaving people without light and heat,” he tweeted.

Ukrainian officials said Russia hit Kharkiv TEC-5, the country’s second-biggest heat and power plant.

Kharkiv Mayor Igor Terekhov called the power outage “revenge by the Russian aggressor for the successes of our army at the front, in particular, in the Kharkiv region.”

The Ukrainian General Staff said Russian forces had left several settlements in the Kherson region as Ukrainian forces pressed the counteroffensive. It did not identify them.

An official with the Russian-backed administration in the city of Kherson, Kirill Stremousov, said on social media that the city was safe and asked everyone to stay calm.

The Russian pullback marked the biggest battlefield success for Ukrainian forces since they thwarted a Russian attempt to seize Kyiv near the start of the war. The Kharkiv campaign came as a surprise to Moscow, which had relocated many of its troops from the region to the south in expectation of a counteroffensive there.

In trying to save face, the Russian Defense Ministry said Saturday the withdrawal from Izyum and other areas was intended to strengthen Moscow’s forces in the neighboring Donetsk region to the south. The explanation was similar to how Russia justified pulling back from Kyiv earlier this year.

Igor Strelkov, who led Russia-backed forces when the separatist conflict in the Donbas erupted in 2014, mocked the Russian Defense Ministry’s explanation of the retreat, suggesting that handing over Russia’s own territory near the border was a “contribution to a Ukrainian settlement.”

The retreat angered Russian military bloggers and nationalist commentators, who bemoaned it as a major defeat and urged the Kremlin to step up its war efforts. Many criticized Russian authorities for continuing with fireworks and other lavish festivities in Moscow that marked a city holiday on Saturday despite the debacle in Ukraine.

Secretary of State Antony Blinken and the head of NATO cautioned Friday the war would likely go on for months, urging the West to keep supporting Ukraine through what could be a difficult winter.