KYIV, Ukraine — An explosion caused by an alleged drone attack at Europe’s largest nuclear plant in Ukraine on Tuesday posed no direct threat to its safety but underscored the “extremely serious situation” at the facility that repeatedly has been caught in the war 's crossfire, the U.N.’s atomic watchdog agency said.

The International Atomic Energy Agency said its team was aware of an explosion at a training center next to the Zaporizhzhia Nuclear Power Plant. It said it was informed the blast was from a drone attack but gave no further details.

The agency's information presumably came from Russians who have occupied and run the plant since the war's early stages.

The Zaporizhzhia facility is one of the 10 biggest nuclear plants in the world. Fighting in the southern part of Ukraine where it is located has raised the specter of a potential nuclear disaster like the one at Chernobyl in 1986, where a reactor exploded and blew deadly radiation across a vast area.

Neither Russia nor Ukraine in recent months has been able to make significant advances along the 1,000-kilometer (620-mile) front line crossing eastern and southern Ukraine. Drones, artillery and missiles have featured heavily in what has become a war of attrition.

Russia and Ukraine have frequently traded accusations over the Zaporizhzhia plant. On Monday, Moscow alleged Ukraine was behind drone attacks on the facility a day before, and Kyiv accused Russia of disinformation tactics .

The IAEA reported Sunday that its inspectors had confirmed “the physical impact of drone detonations” and watched as “Russian troops engaged what appeared to be an approaching drone.”

Energoatom, the Ukrainian nuclear plant operator, blasted Russia’s latest allegations in a statement Tuesday. It accused Moscow of publishing propaganda and “false statements” to manipulate public opinion against Ukraine.

Energoatom noted that Russia has deployed troops and landmines at the site, which is one of four atomic power plants in Ukraine. The other three remain in Ukrainian hands.

“The dangerous game of the (Russian) occupiers at the (Zaporizhzhia) nuclear facility must be stopped,” it said.

The most recent strikes did not compromise the facility, which is designed to withstand a commercial airliner crashing into it, the IAEA said. But the watchdog has repeatedly expressed alarm about the plant amid fears of a nuclear catastrophe.

The plant’s six reactors have been shut down for months, but it still needs power and qualified staff to operate crucial cooling systems and other safety features.

Oleksandr Kharchenko, director of the Kyiv-based Energy Industry Research Center, said it made no sense for Ukrainian forces to strike the Zaporizhzhia plant because the country will need the energy it produces.

Russian forces recently renewed their efforts to pound the Ukrainian power grid, using improved intelligence and tactics, Ukraine says.

“The main point for the Ukrainian side right now, especially in the situation where we are right now, is to save Zaporizhzhia nuclear station, because for our energy systems, this station is a critical game-changer,” Kharchenko said.

According to Ukrainian emergency services, a nuclear disaster would compel the evacuation of some 300,000 people.

Also Tuesday, Ukraine’s intelligence agency claimed it struck an aviation training center in southwestern Russia with a drone and said a fire on board a Russian navy corvette on the Baltic coast was “not accidental.”

The claims could not be independently verified or corroborated. Russia made no comment on them.