KYIV, Ukraine — Ukrainian troops on Sunday successfully pressed their swift counteroffensive in the northeastern part of the country, even as a nuclear power plant in the Russia-occupied south completely shut down in a bid to prevent a radiation disaster as fighting raged nearby.

But Russia struck back at Ukraine’s infrastructure Sunday night, causing widespread blackouts, with the Kharkiv and Donetsk regions among those without power, officials said.

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 in a hasty retreat as the war marked its 200th day on Sunday.

Ukrainian President Volodymyr Zelenskyy 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.

While most attention focused on the counteroffensive, Ukraine’s nuclear energy operator said the Zaporizhzhia nuclear power plant, Europe’s largest, was reconnected to Ukraine’s electricity grid, allowing engineers to shut down its last operational reactor to safeguard it amid the fighting.

The plant, one of the 10 biggest atomic power stations in the world, has been occupied by Russian forces since the early days of the war. Ukraine and Russia have traded blame for shelling around it.

Since a Sept. 5 fire caused by shelling knocked the plant off transmission lines, the reactor was powering crucial safety equipment in so-called “island mode” — an unreliable regime that left the plant increasingly vulnerable to a potential nuclear accident.