KYIV, Ukraine (AP) — 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.

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.

A jubilant 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 was 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 the plant 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.

The International Atomic Energy Agency, the U.N. nuclear watchdog that has two experts at the plant, welcomed the restoration of external power. But the agency’s director-general, Rafael Grossi, said he remains “gravely concerned about the situation at the plant, which remains in danger as long as any shelling continues.”

He said talks have begun on establishing a safety and security zone around the plant.

In a call Sunday with Russian President Vladimir Putin, French President Emmanuel Macron urged the withdrawal of Russian troops and weaponry from the plant in line with IAEA recommendations.

In fighting, Ukraine’s military chief, Gen. Valerii Zaluzhnyy, said its forces had recaptured about 3,000 square kilometers (1,160 square miles) since the counteroffensive began in early September. He said Ukrainian troops are only 50 kilometers (about 30 miles) from the border with Russia.

Kharkiv Gov. Oleh Syniehubov said Ukrainian troops have reclaimed control of more than 40 settlements in the region, noting he couldn’t give a precise number because the operation is still unfolding.

Defense Minister Anna Malyar said Ukrainian forces are firing shells containing propaganda into areas where they seek to advance.

”One of the ways of informational work with the enemy in areas where there is no Internet is launching propaganda shells,” she wrote on Facebook. “Before moving forward, our defenders say hello to the Russian invaders and give them the last opportunity to surrender. Otherwise, only death awaits them on Ukrainian soil.”

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

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 the capital, Kyiv, near the start of the war. The Kharkiv campaign came as a surprise for Moscow, which had relocated many of its troops from the region to the south in expectation of a counteroffensive there.

In an awkward attempt to save face, the Russian Defense Ministry said Saturday the troops’ withdrawal from Izyum and other areas was intended to strengthen Moscow’s forces in the neighboring Donetsk region to the south. The explanation sounded similar how Russia justified its 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 drew an angry response from 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.

Putin attended the opening of a huge Ferris wheel in a Moscow park on Saturday, and inaugurated a new transport link and a sports arena. The action underlined the Kremlin’s narrative that the war it calls a “special military operation” was going according to plan without affecting Russians’ everyday lives.

Pro-Kremlin political analyst Sergei Markov criticized the Moscow festivities as a grave mistake.

“The fireworks in Moscow on a tragic day of Russia’s military defeat will have extremely serious political consequences,” Markov wrote on his messaging app channel. “Authorities mustn’t celebrate when people are mourning.”

In a sign of a potential rift in the Russian leadership, Ramzan Kadyrov, the Kremlin-backed head of Chechnya, said the retreat resulted from blunders by the Russian brass.

“They have made mistakes and I think they will draw the necessary conclusions,” Kadyrov said. “If they don’t make changes in the strategy of conducting the special military operation in the next day or two, I will be forced to contact the leadership of the Defense Ministry and the leadership of the country to explain the real situation on the ground.”

Despite Ukraine’s gains, U.S. Secretary of State Antony Blinken and the head of NATO warned Friday the war would likely drag on for months, urging the West to keep supporting Ukraine through what could be a difficult winter.

Sen. Tim Kaine, a Virginia Democrat and member of the Senate Foreign Relations Committee, called Ukrainian advances very encouraging.

“I’m proud that the U.S. and our allies have locked arms to support the Ukrainian people in this fight,” Kaine said in a statement to The Associated Press. “We and our allies must keep standing with Ukraine. Putin needs to recognize that the only way out is to end his failed war.”