KYIV, Ukraine (NewsNation Now) — Russian forces shelled, then apparently seized, Europe’s largest nuclear plant early Friday, sparking a fire as they pressed their attack on a crucial energy-producing Ukrainian city and gaining ground in their bid to cut off the country from the sea.

The world’s leading nuclear authorities were concerned — but not panicked — about the damage to the power station, but the assault triggered a phone call between U.S. President Joe Biden and Ukrainian President Volodymyr Zelenskyy, and the U.S. Department of Energy activated its nuclear incident response team as a precaution.

Both leaders have urged Russian President Vladimir Putin to “cease its military activities in the area and allow firefighters and emergency responders to access the site,” according to the White House. According to the British Broadcasting Corporation, the Ukranian military announced early Friday that the Russians had captured the plant.

The attack on the eastern city of Enerhodar and its Zaporizhzhia nuclear plant unfolded as the invasion entered its second week and another round of talks between the two sides yielded a tentative agreement to set up safe corridors to evacuate citizens and deliver humanitarian aid.

U.S. Energy Secretary Jennifer Granholm tweeted that the Zaporizhzhia plant’s reactors were protected by robust containment structures and were being safely shut down.

Russia is continuing its efforts to occupy Ukraine’s second-largest city and two strategic seaports, which the Kremlin claims are threatening Russia. A member of Ukraine’s delegation in talks with Russia said both parties have reached a tentative agreement to organize safe corridors for civilians to evacuate and for humanitarian supplies to be delivered. Russia and Ukraine reached a preliminary understanding that cease-fires will be observed in areas where safe corridors are established, said Mykhailo Podolyak, an adviser to Zelenskyy who took part in Thursday’s talks.

On Thursday, Zelenskyy called for Putin to meet with him.

“Sit down with me to negotiate,” he said. “I don’t bite. What are you afraid of?”

U.S. officials say Russia has fired 480 missiles at Ukraine as Russian troops make more progress in the south, but are largely stalled in the north.

With a column of Russian tanks and other vehicles apparently stalled for days outside the capital of Kyiv, fighting continued on multiple fronts across Ukraine.

Russian troops gained control of Kherson on Thursday, the first Ukrainian city to be overcome during the war.

Russia currently has the following cities under siege: Kharkiv, Mariupol, Chernihiv, Enerhodar and Lviv, among others.

In Chernihiv, a city of 280,000 in Ukraine’s north, at least 33 civilians were killed and an additional 18 wounded in a Russian strike, Ukraine’s state emergencies agency said Thursday.

Video taken in the aftermath of the shelling shows firefighters standing in rubble dousing flames with hoses as rescue crews carried at least one person on a stretcher and another helper assisted a person down a ladder.

Smoke spewed from a high-rise building just behind what appeared to be a children’s swing set, according to video released Thursday by the Ukrainian government.

Britain’s Defense Ministry said Mariupol, a large city on the Sea of Azov, was encircled by Russian forces.

“Deliberately, for seven days, they have been destroying (Mariupol’s) critical life-support infrastructure. We have no light, water or heat again,” the Mariupol city council said. “Mariupol remains under fire. Women, children and the elderly are suffering. We are being destroyed as a nation. This is genocide of the Ukrainian people.”

Russia’s Defense Ministry said it had knocked out a reserve broadcasting center in the Lysa Hora district, about 4 miles south of the government headquarters. It said unspecified precision weapons were used, and that there were no casualties or damage to residential buildings.

A statement from the general staff of Ukraine’s armed forces didn’t address the strikes, saying only that Russian forces were “regrouping” and “trying to reach the northern outskirts” of the city.

To date, at least 249 civilians have died and another 553 have been wounded since the invasion began, according to the latest figures from the office of the U.N. High Commissioner for Human Rights. Seventeen of those killed were children.

In the days since the invasion, more than 1 million people have fled Ukraine following Russia’s invasion in the swiftest refugee exodus this century, the United Nations said.

The tally by the U.N. refugee agency amounts to more than 2% of Ukraine’s population being forced out of the country in seven days. The mass evacuation could be seen in Kharkiv, a city of about 1.5 million people where residents desperate to escape falling shells and bombs crowded the city’s train station and pressed onto trains, not always knowing where they were headed.

The U.S. and its allies have insisted that NATO is a defensive alliance that doesn’t pose a threat to Russia. And the West fears Russia’s invasion is meant to overthrow Ukraine’s government and install a friendly government.

Zelenskyy called on Ukrainians to keep up the resistance in a Thursday video address.

“We are a people who in a week have destroyed the plans of the enemy,” he said. “They will have no peace here. They will have no food. They will have here not one quiet moment.”

As the invasion grows, Russia is finding itself increasingly isolated economically. The ruble, which has tanked since the invasion, lost a further 15% against the dollar.

Attorney General Merrick Garland announced Wednesday the creation of Task Force KleptoCapture, a team of federal agents and prosecutors responsible for investigating and prosecuting any violations of new and future sanctions against Russia.

That includes seizing the assets belonging to oligarchs and others who violate the sanctions, and thwarting any efforts to use cryptocurrency to get around sanctions.