A 14-strong team of inspectors from the International Atomic Energy Agency arrived at the facility on Thursday to conduct “security and safeguards activities” after a risky journey across the frontline and early-morning shelling of the area.

“It is obvious that the plant and physical integrity of the plant has been violated several times,” IAEA head Rafael Grossi told reporters after returning to Ukrainian-controlled territory.

Grossi said part of the IAEA mission would stay at Zaporizhzhia “until Sunday or Monday” to continue the assessment, without specifying their number.

The Argentine described the visit as productive and said he gathered lots of information.

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After the inspection, in a video released by the Russian RIA Novosti news agency, Grossi said: “We have achieved something very important today and the important thing is the IAEA is staying here”.

Russia’s ambassador to international institutions in Vienna said two IAEA inspectors would stay at the plant on a permanent basis, RIA said on Friday.

A dawn shelling attack on the area had forced one of the plant’s six reactors to close.

Energoatom, Ukraine’s nuclear agency, said it was “the second time in 10 days” that Russian shelling had forced the closure of a reactor.

It said the plant’s emergency protection system kicked in shortly before 5am, shutting reactor five, with the attack damaging a backup power supply.

Energoatom also said it would be would be difficult for the IAEA team to make an impartial assessment due to Russian interference.

“The Russians did not allow the mission to enter the crisis centre, where Russian military personnel are currently stationed, whom the IAEA representatives were not supposed to see,” Energoatom said in a statement.

“It is high time to stop playing with fire and instead take concrete measures to protect this facility … from any military operations,” Robert Mardini, chief of the International Committee of the Red Cross, told reporters in Kyiv.

He warned the consequences of hitting the plant could be “catastrophic”, saying “the slightest miscalculation could trigger devastation that we will regret for decades”.

After Russian forces seized the plant on March 4, Energoatom shut two reactors, followed by a third after shelling on August 5. With a fourth undergoing repairs, Thursday’s incident leaves only one of the six reactors working.

Mardini said it was “encouraging” the IAEA team was inspecting the plant because the stakes were “immense”.

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On leaving Zaporizhzhia, Grossi said his team would be travelling through areas where “the risks are significant” but had decided to go ahead anyway due to the “very important mission to accomplish”.

The neighbouring town of Energodar came under sustained attack at dawn, with Russian troops firing “mortars and using automatic weapons and rockets”, its mayor Dmytro Orlov said.

But Moscow accused Kyiv of smuggling in up to 60 military “saboteurs” who reached the area near the plant at dawn, prompting Russian troops to take “measures to annihilate the enemy”.

Ukraine has accused Russia of deploying hundreds of soldiers and storing ammunition at the plant. Russian Defence Minister Sergei Shoigu rejected that assertion, saying Kyiv was “creating a real threat of nuclear catastrophe” and using Western-supplied weapons to attack the plant.

Shoigu accused the United States and European Union of “encouraging such reckless actions”.

Kyiv also suspects that Moscow intends to divert power from the plant to the nearby Crimean peninsula, annexed by Russia in 2014 – a view held by other international figures including British Prime Minister Boris Johnson.

Meanwhile, Ukrainian troops pressed ahead with a counteroffensive in the nearby region of Kherson to retake areas seized by Russia at the start of the invasion.

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Both sides have claimed battlefield successes in the new Ukrainian push, although details have been scarce so far, with Ukrainian officials releasing little information about their advance.

“It is a very slow process, because we value people,” said Oleksiy Arestovych, an adviser to Ukraine President Volodymyr Zelensky. Moscow has denied reports of Ukrainian progress and said its troops had routed Ukrainian forces.

Ukraine’s military said late on Thursday that Russian forces had shelled around a dozen southern towns, with attacks in other regions including Kharkiv in the north and Donetsk in the east.

Additional reporting by Reuters