NIKOPOL, Ukraine — The Zaporizhzhia nuclear power plant in the middle of the fighting in Ukraine was temporarily knocked offline Thursday because of fire damage to a transmission line, causing a blackout across the region and heightening fears of a catastrophe in a country still haunted by the Chernobyl disaster.

The complex, Europe's largest nuclear plant, has been occupied by Russian forces and run by Ukrainian worker s since the early days of the 6-month-old war. Ukraine alleges Russia is essentially holding the plant hostage, storing weapons there and launching attacks from around it, while Moscow accuses Ukraine of recklessly firing on the facility.

On Thursday, the plant was cut off from the electrical grid after fires damaged the last operating regular transmission line, according to Ukraine’s nuclear power agency, Energoatom. Ukrainian President Volodymyr Zelenskyy blamed Russian shelling and said the plant’s emergency backup diesel generators had to be activated to supply power needed to run the plant.

“Russia has put Ukraine and all Europeans one step away from a radiation disaster,” Zelenskyy said in his nightly video address.

Zaporizhzhia's Russian-installed regional governor, Yevgeny Balitsky, blamed the transmission-line damage on a Ukrainian attack.

It was not immediately clear whether the damaged line carried outgoing electricity or incoming power, needed for the reactors' vital cooling systems. A backup line supplying electricity from another plant remained in place, Energoatom said.

But Zelenskyy’s mention of the emergency generators being activated raised questions of whether the cooling systems were endangered. A loss of cooling could cause a nuclear meltdown.

As a result of the transmission-line damage, the two reactors still in use out of the plant's six went offline, Balitsky said, but one was quickly restored, as was electricity to the region.

Many nuclear plants are designed to automatically shut down or at least reduce reactor output in the event of a loss of outgoing transmission lines. The U.N.'s International Atomic Agency said Ukraine informed it that the reactors' emergency protection systems were triggered, and all safety systems remained operational.

The three regular transmission lines at the plant are out of service because of previous war damage.

“Anybody who understands nuclear safety issues has been trembling for the last six months,” Mycle Schneider, a consultant and coordinator of the World Nuclear Industry Status Report, said before the latest incident.

Ukraine cannot simply shut down its nuclear plants during the war because it is heavily reliant on them. Its 15 reactors at four stations provide about half of its electricity. Still, an armed conflict near a working atomic plant is troubling for many experts and people living nearby.

That fear is palpable just across the Dnieper River in Nikopol, where residents have been under nearly constant Russian shelling since July 12, with eight people killed, 850 buildings damaged and over half the population of 100,000 fleeing the city.

Liudmyla Shyshkina, a 74-year-old widow who lived within sight of the Zaporizhzhia plant before her apartment was bombarded and her husband killed, said she believes the Russians are capable of intentionally causing a nuclear disaster.

Zelenskyy has accused Russia of “nuclear blackmail" at Zaporizhzhia. F ighting in early March caused a brief fire at the plant’s training complex that officials said did not result in the release of any radiation.

While no civilian nuclear plant is designed for a wartime situation, Zaporizhzhia’s reactors are protected by reinforced concrete containment domes that could withstand an errant shell, experts say.

The more immediate concern is that a disruption in the electrical supply could knock out cooling systems essential for the reactors' safe operation. Emergency diesel generators can be unreliable.

The pools where spent fuel rods are kept while they cool are also vulnerable to shelling, which could scatter radioactive material.

Kyiv told the IAEA that shelling earlier this week damaged transformers at a nearby conventional power plant, disrupting the supplies of electricity to the Zaporizhzhia plant for several hours.

The atomic agency's head, Rafael Mariano Grossi, said Thursday he hopes to send a team to the plant within days.

Negotiations over how the team would access the plant are complicated but advancing, he said on France-24 television.

“Kyiv accepts it. Moscow accepts it. So we need to go there," Grossi said.

At a U.N. Security Council meeting Tuesday, U.N. political chief Rosemary DiCarlo urged the withdrawal of all troops and military equipment from the plant and an agreement on a demilitarized zone around it.

Speaking before Thursday's incident, one expert explained that power is essential to cool not just the reactors but also the spent radioactive fuel.

“If we lose the last one, we are at the total mercy of emergency power generators,” said Najmedin Meshkati, a professor of civil and environmental engineering at the University of Southern California.

He and Schneider expressed concern that the occupation of the plant by Russian forces is also hampering safety inspections and the replacement of critical parts, and is putting severe strain on hundreds of Ukrainian staff who operate the facility.

“Human error probability will be increased manifold by fatigue,” said Meshkati, part of a committee appointed by the U.S. National Academy of Sciences to identify lessons from the 2011 nuclear disaster at Japan's Fukushima plant. “Fatigue and stress are unfortunately two big safety factors.”

If an incident at the Zaporizhzhia plant were to release radiation, the scale of the crisis would be determined largely by the winds and other weather conditions.

The massive earthquake and tsunami that hit the Fukushima plant destroyed cooling systems, triggering meltdowns in three of its reactors. Much of the contaminated material was blown out to sea, limiting the damage.

The April 26, 1986, explosion and fire at one of four reactors at the Chernobyl plant north of Kyiv sent a cloud of radioactive material across a wide swath of Europe and beyond.

Zaporizhzhia's reactors are of a different model from those at Chernobyl, but unfavorable winds could still spread radioactive contamination in any direction, said Paul Dorfman, a nuclear safety expert at the University of Sussex who has advised the British and Irish governments.

"If something really went wrong, then we have a full-scale radiological catastrophe that could reach Europe, go as far as the Middle East, and certainly could reach Russia, but the most significant contamination would be in the immediate area,” he said.

That's why Nikopol's emergency services department has been taking radiation measurements every hour since the Russian invasion. Before that, it was every four hours.