Russian forces shelled Europe's largest nuclear plant early Friday, sparking a fire as they pressed their attack on a crucial energy-producing Ukrainian city and gained ground in their bid to cut off the country from the sea.

Leading nuclear authorities were concerned -- but not panicked -- about the damage to the power station. The assault triggered phone calls between Ukrainian President Volodymyr Zelenskyy and US President Joe Biden and other world leaders. The US Department of Energy activated its nuclear incident response team as a precaution.

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.

Nuclear plant spokesman Andriy Tuz told Ukrainian television that shells were falling directly on the facility and had set fire to one of its six reactors. That reactor is under renovation and not operating, but there is nuclear fuel inside, he said.

Firefighters cannot get near the flames because they are being shot at, he said, and Ukrainian Foreign Minister Dmytro Kuleba tweeted a plea to the Russians to stop the assault and allow fire teams inside.

"We demand that they stop the heavy weapons fire," Tuz said in a video statement. "There is a real threat of nuclear danger in the biggest atomic energy station in Europe."

The assault renewed fears that the invasion could damage one of Ukraine's 15 nuclear reactors and set off another emergency like the 1986 Chernobyl accident, the world's worst nuclear disaster, which happened about 110 kilometers (65 miles) north of the capital.

US 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."

"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."

But most experts saw nothing to indicate an impending disaster.

The International Atomic Energy Agency 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.

Jon Wolfsthal, who served during the Obama administration as the senior director for arms control and nonproliferation at the National Security Council, said the plant's reactors have thick concrete containment domes that should protect them from tank and artillery fire.

But he too was concerned about a potential loss of power at the plant, which could imperil its ability to keep the nuclear fuel cool.

The mayor of Enerhodar said earlier that Ukrainian forces were battling Russian troops on the city's outskirts. Video showed flames and black smoke rising above the city of more than 50,000, with people streaming past wrecked cars.

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.

"Many young men in athletic clothes and armed with Kalashnikovs have come into the city. They are breaking down doors and trying to get into the apartments of local residents," the statement from Energoatom said.

Later, a livestreamed 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.

Ukrainian Prime Minister Denys Shmyhal called on the West to close the skies over the country's nuclear plants. "It is a question of the security of the whole world!" he said in a statement.

The US and NATO allies have ruled out creating a no-fly zone since the move would pit Russian and Western military forces against each other.

Vladimir Putin's 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.

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 would deal a crippling blow to its economy and allow 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 US 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 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 safe corridors, and that they would seek to work out the necessary details quickly. A Zelenskyy adviser also said a 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.

He also hailed Russian soldiers as heroes in a video call with members of Russia's Security Council, and ordered additional payments to families of men killed or wounded.

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 US defense official who spoke on condition of anonymity because the link had not been announced. (AP)