A fire at Europe’s biggest nuclear plant ignited by Russian shelling has been extinguished, Ukrainian authorities said Friday, and Russian forces have gained full control of the site.

Ukraine’s state nuclear regulator said that no changes in radiation levels have been recorded so far. It said staff is studying the site to check for other damage to the compartment of reactor No. 1 at the Zaporizhzhia plant in the city of Enerhodar.

"At 6:20 a.m. (4:20 a.m. GMT) the fire in the training building of Zaporizhzhia NPP in Enerhodar was extinguished. There are no victims," the emergency services said in a statement on Facebook.

The regulator noted in a statement on Facebook the importance of maintaining the ability to cool nuclear fuel, saying the loss of such ability could lead to an accident even worse than the 1986 Chernobyl accident, the world’s worst nuclear disaster, or the 2011 Fukushima meltdowns in Japan. It also noted that there is a storage facility for spent nuclear fuel at the site, though there was no sign that the facility was hit by shelling.

The shelling of the plant came as the Russian military pressed their attack on a crucial energy-producing Ukrainian city and gained ground in their bid to cut off the country from the sea. As the invasion entered its second week, another round of talks between Russia and Ukraine yielded a tentative agreement to set up safe corridors to evacuate citizens and deliver humanitarian aid.

Leading nuclear authorities were worried – but not panicked – about the damage to the power station. The assault, however, led to phone calls between Ukrainian President Volodymyr Zelenskyy and U.S. President Joe Biden and other world leaders. The U.S. Department of Energy activated its nuclear incident response team as a precaution.

Earlier, nuclear plant spokesperson Andriy Tuz told Ukrainian television that shells fell directly on the facility and set fire to one of its six reactors. That reactor is under renovation and not operating, he said.

The Zaporizhzhia regional military administration said that measurements were taken at 7 a.m. Friday (5 a.m. GMT) showed radiation levels in the region "remain unchanged and do not endanger the lives and health of the population."

The mayor of Enerhodar, Dmytro Orlov, announced on his Telegram channel Friday morning that "the fire at the (nuclear plant) has indeed been extinguished." His office told The Associated Press (AP) that the information came from firefighters who were allowed onto the site overnight.

British Prime Minister Boris Johnson called for an emergency meeting of the U.N. Security Council in "coming hours" to raise the issue of Russia’s attack on the nuclear power plant, according to a statement from his office.

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.

In an emotional speech in the middle of the night, Zelenskyy said he feared an explosion that would be "the end for everyone. The end for Europe. The evacuation of Europe." Zelenskyy accused Moscow of resorting to "nuclear terrorism" and wanting to "repeat" the Chernobyl disaster after he said invading Russian forces deliberately attacked the nuclear power plant.

"Only urgent action by Europe can stop the Russian troops," he said. "Do not allow the death of Europe from a catastrophe at a nuclear power station."

The station at Zaporizhzhia, an industrial city in the southeast, supplies an estimated 40% of the country's nuclear power and, according to Zelenskyy, houses six of Ukraine's 15 reactors. The Zaporizhzhia plant is located in the southern Ukraine steppe on the Dnieper River, around 525 kilometers (325 miles) south of Chernobyl.

"This is the first time in our history. In the history of mankind. The terrorist state now resorted to nuclear terrorism," Zelenskyy said, adding that Russian troops had knowingly fired on the nuclear facility. "These are tanks equipped with thermal imagers, so they know where they are shooting," said Zelenskyy.

But most experts saw nothing to indicate an impending disaster.

The International Atomic Energy Agency (IAEA) said the fire had not affected essential equipment and that Ukraine’s nuclear regulator reported no change in radiation levels. The American Nuclear Society concurred, saying that the latest radiation levels remained within natural background levels.

"The real threat to Ukrainian lives continues to be the violent invasion and bombing of their country," the group said in a statement.

NATO Secretary-General Jens Stoltenberg denounced the attack, saying that it shows the recklessness of its invasion. The head of the military alliance called on Russia to withdraw its troops from Ukraine and engage in diplomatic efforts to resolve the conflict. Stoltenberg was speaking with U.S. Secretary of State Antony Blinken at a meeting of NATO foreign ministers in Brussels on Friday.

Orlov, the mayor of Enerhodar, said Russian shelling stopped a few hours before dawn, and residents of the city of more than 50,000 who had stayed in shelters overnight could return home. The city woke up to no heating because the shelling damaged the city’s heating main, he said.

Prior to the shelling, the Ukrainian state atomic energy company reported that a Russian military column was heading toward the nuclear plant. Loud shots and rocket fire were heard late Thursday.

Later, a live-streamed security camera linked from the homepage of the Zaporizhzhia plant showed what appeared to be armored vehicles rolling into the facility’s parking lot and shining spotlights on the building where the camera was mounted.

Then there were what appeared to be muzzle flashes from vehicles, followed by nearly simultaneous explosions in surrounding buildings. Smoke rose into the frame and drifted away.

Russian forces have brought their superior firepower to bear over the past few days, launching hundreds of missiles and artillery attacks on cities and other sites around the country and making significant gains in the south.

The Russians announced the capture of the southern city of Kherson, a vital Black Sea port of 280,000, and local Ukrainian officials confirmed the takeover of the government headquarters there, making it the first major city to fall since the invasion began a week ago.

A Russian airstrike on Thursday destroyed the power plant in Okhtyrka, leaving the city without heat or electricity, the head of the region said on Telegram. In the first days of the war, Russian troops attacked a military base in the city, located between Kharkiv and Kyiv, and officials said more than 70 Ukrainian soldiers were killed.

"We are trying to figure out how to get people out of the city urgently because in a day the apartment buildings will turn into a cold stone trap without water, light or electricity," Dmytro Zhyvytskyy said.

Heavy fighting continued on the outskirts of another strategic port, Mariupol, on the Azov Sea. The battles have knocked out the city’s electricity, heat and water systems, as well as most phone service, officials said. Food deliveries to the city were also cut.

Associated Press video from the port city showed the assault lighting up the darkening sky above deserted streets and medical teams treating civilians, including a 16-year-old boy inside a clinic who could not be saved. The child was playing soccer when he was wounded in the shelling, according to his father, who cradled the boy’s head on the gurney and cried.

Severing Ukraine’s access to the Black and Azov seas dealt a crippling blow to its economy and allowed Russia to build a land corridor to Crimea, seized by Moscow in 2014.

Overall, the outnumbered, outgunned Ukrainians have put up stiff resistance, staving off the swift victory that Russia appeared to have expected. But a senior U.S. defense official, speaking on condition of anonymity, said Russia’s seizure of Crimea gave it a logistical advantage in that part of the country, with shorter supply lines that smoothed the offensive there.

Ukrainian leaders called on the people to defend their homeland by cutting down trees, erecting barricades in the cities and attacking enemy columns from the rear. In recent days, authorities have issued weapons to civilians and taught them how to make Molotov cocktails.

"Total resistance ... This is our Ukrainian trump card, and this is what we can do best in the world," Oleksiy Arestovich, an aide to Zelenskyy, said in a video message, recalling guerrilla actions in Nazi-occupied Ukraine during World War II.

The second round of talks between Ukrainian and Russian delegations was held in neighboring Belarus. But the two sides appeared far apart going into the meeting, and Putin warned Ukraine that it must quickly accept the Kremlin’s demand for its "demilitarization" and declare itself neutral, renouncing its bid to join NATO.

Putin told French President Emmanuel Macron that he was determined to press on with his attack "until the end," according to Macron’s office.

The two sides said that they tentatively agreed to allow cease-fires in areas designated as safe corridors and that they would seek to work out the necessary details quickly. A Zelenskyy adviser also said the third round of talks will be held early next week.

Despite a profusion of evidence of civilian casualties and destruction of property by the Russian military, Putin decried what he called an "anti-Russian disinformation campaign" and insisted that Moscow uses "only precision weapons to exclusively destroy military infrastructure."

Putin claimed that the Russian military had already offered safe corridors for civilians to flee, but he asserted without evidence that Ukrainian "neo-Nazis" were preventing people from leaving and were using them as human shields.

The Pentagon set up a direct communication link to Russia’s Ministry of Defense earlier this week to avoid the possibility of a miscalculation sparking conflict between Moscow and Washington, according to a U.S. defense official who spoke on condition of anonymity because the link had not been announced.