United Nations inspectors made their way toward Ukraine's Zaporizhzhia nuclear power plant on Wednesday, a long-anticipated mission that the world hopes will help secure the Russian-held facility in the middle of a war zone and avoid catastrophe.

Underscoring the danger, Kyiv and Moscow again accused each other of attacking the area around Europe's biggest nuclear plant.

In recent days, the plant was temporarily knocked offline because of fire damage to transmission line — heightening fears that fighting could lead to a massive radiation leak or even a reactor meltdown. The risks are so severe that officials have begun distributing anti-radiation iodine tablets to nearby residents.

Russia-backed local authorities claimed Wednesday that Ukrainian forces repeatedly shelled the territory of the plant and that drone strikes hit the plant’s administrative building and training center. Regional Ukraine governor Valentyn Reznichenko, meanwhile, said a city across the river from the plant came under heavy artillery fire during the night.

“This appears to be nuclear blackmail of the local population and international society,” Reznichenko said on Telegram.

The complex, a vital source of energy for Ukraine, has been occupied by Russian forces and run by Ukrainian workers 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.

For months, as the fighting has played out, the head of the International Atomic Energy Agency has sought access to the plant — and world leaders have demanded the U.N. nuclear watchdog be allowed to inspect it.

With a team finally on the way, Rafael Grossi, the head of the agency, said he knew full well the implications of the unprecedented mission.

“We are going to a war zone. We are going to occupied territory,” he said upon departure early Wednesday.

He added that he had received “explicit guarantees” from Russia that the mission of 14 experts would be able to do its work.

Russian authorities in Enerhodar, where the plant is located, said there were no casualties or release of radioactivity in the most recent fighting.

Read:U.N. monitors head to troubled Ukraine nuclear plant

But that did little to assuage fears for the safety of the U.N. mission itself. Ukraine on Tuesday accused the Russians of bombing the roads the mission planned to use to access the plant, alleging they were trying to encourage the inspectors to course route and move via Russia-controlled areas instead.

The world watched the mission's progress with anxiety. European Union foreign policy chief Josep Borrell renewed a call to Russia for the area around the power plant to be fully demilitarized.

“They are playing games. They are gambling with the nuclear security,” Borrell told reporters in the Czech capital, Prague. “We cannot play war games in the neighborhood of a site like this.”

Kyiv is seeking international assistance to take back control of the area.

“We think that the mission should be a very important step to return (the plant) to Ukrainian government control by the end of the year,” Ukrainian Energy Minister German Galushchenko told The Associated Press.

If all goes well, the inspectors should reach the Zaporizhzhia region, 450 kilometers (280 miles) southeast of the Ukrainian capital, later Wednesday. The experts may have to pass through areas of active fighting, with no publicly announced cease-fire.

Grossi met Tuesday with Ukrainian President Volodymyr Zelenskyy to discuss the mission, which is expected to last several days.

In other developments:

— Zelenskyy's office said Wednesday that automatic weapons fire was heard on the streets of southern Kherson and claimed Russian soldiers were searching private residences for partisans, as Ukrainians resisting Russian rule are known. There was speculation early in the week that Ukraine had attempted to start a counteroffensive there.

His office said that in the east, four people were killed and two wounded in rocket firing in the Donetsk region in the past day.

— Russia’s Gazprom stopped the flow of natural gas through a major pipeline from to western Europe early Wednesday, a move it announced in advance and said was for routine maintenance.