Europe's largest nuclear power station is on fire following an attack by Russian troops.

U.S. President Joe Biden spoke with President Volodymyr Zelenskyy of Ukraine and has also urged Russia to stop military activities in the area of the Zaporizhzhia Nuclear Power Plant so that firefighters and emergency responders can access the site and put out the flames.

President Biden has also been in touch with the Under Secretary for Nuclear Security of the U.S. Department of Energy and Administrator of the National Nuclear Security Administration to be updated on the situation at the nuclear plant.

Ukraine's foreign minister has called for a security zone, and also pressed the importance of immediately allowing firefighters to be allowed to go in and handle the blaze, Reuters reported.

The fire at the nuclear power station in the city of Enerhodar comes after Russian troops were shelling it just before, Ukraine's foreign minister said.

“We demand that they stop the heavy weapons fire,” Andriy Tuz, spokesperson for the plant in Enerhodar, said in a video posted on Telegram. “There is a real threat of nuclear danger in the biggest atomic energy station in Europe.”

Ukraine's Foreign Affairs Minister Dmytro Kuleba tweeted late on Thursday, "Fire has already broke out. If it blows up, it will be 10 times larger than Chornobyl! Russians must IMMEDIATELY cease the fire, allow firefighters, establish a security zone!"

The plant accounts for about one quarter of Ukraine’s power generation.

U.S. Secretary of Energy Jennifer Granholm said she has spoken with Ukraine's energy minister about this situation and called on Russian troops to cease their attack calling the situation "reckless." She said the U.S. Department of Energy has activated its Nuclear Incident Response Team and is monitoring the situation along with the U.S. Department of Defense and the U.S. Nuclear Regulatory Commission.

The fighting at Enerhodar, a city on the Dnieper River, came as another round of talks between the two sides yielded a tentative agreement to set up safe corridors inside Ukraine to evacuate citizens and deliver humanitarian aid.

The mayor of Enerhodar said 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, just a day after the U.N. atomic watchdog agency expressed grave concern that the fighting could cause accidental damage to Ukraine’s 15 nuclear reactors.

Earlier, both the Ukrainian state atomic energy company and the mayor of Enerhodar, Dmytro Orlov, said Russian troops were approaching the Zaporizhzhia nuclear plant. Officials said loud shots were heard in the city late Thursday.

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

Ukrainian Prime Minister Denys Shmyhal has joined Ukraine’s president in calling on the West to close the skies over Ukraine’s nuclear plants as fighting intensified around the major energy hub on the left bank of the Dnieper River and the Khakhovka Reservoir.

Shmyhal said he already had appealed to NATO and the International Atomic Energy Agency, the United Nations' atomic watchdog.

“Close the skies over Ukraine! It is a question of the security of the whole world!” Shmyhal said in a statement Thursday evening.

The U.S. and NATO allies have ruled out creating a no-fly zone since the move would directly pit Russian and Western militaries.

BERLIN — The United Nations’ atomic watchdog says Ukraine has informed the International Atomic Energy Agency that staff who have been kept at the Chernobyl Nuclear Power Plant since Russian troops took control of the site a week ago are facing “psychological pressure and moral exhaustion.”

IAEA Director General Rafael Mariano Grossi said Thursday that the staff must be allowed to rest and rotate so their crucial work can be carried out safely and securely.

Grossi received “a joint appeal from the Ukraine Government, regulatory authority and the national operator which added that personnel at the Chornobyl site ‘have limited opportunities to communicate, move and carry out full-fledged maintenance and repair work,’” the IAEA said in a statement.

Reactor No. 4 at the power plant exploded and caught fire in 1986, shattering the building and spewing radioactive material high into the sky. Even 36 years later, radioactivity is still leaking from history’s worst nuclear disaster.

Ukraine has lost regulatory control over all the facilities in the Chernobyl Exclusion Zone to the Russians and asked the IAEA to undertake measures “in order to reestablish legal regulation of safety of nuclear facilities and installations” within the site, the statement added.

Grossi has repeatedly stressed that any military or other action that could threaten the safety or security of Ukraine’s nuclear power plants must be avoided.

“I remain gravely concerned about the deteriorating situation in Ukraine, especially about the country’s nuclear power plants, which must be able to continue operating without any safety or security threats,” he said. “Any accident caused as a result of the military conflict could have extremely serious consequences for people and the environment, in Ukraine and beyond.”