Moscow's troops seized Europe's largest nuclear power station shortly after its invasion and later barred Ukrainian staff who refused to take Russian nationality or sign Russian contracts. The facility close to the front line has since become the target of repeated strikes.

Staff from the UN nuclear watchdog have been based at the plant since September 2022 to monitor its safety, with teams rotating regularly. Until now they have accessed the area via Ukraine-controlled territory.

Over the weekend, a monitoring mission by the International Atomic Energy Agency (IAEA) travelled through Russian-occupied territory for the first time.

Ukraine's foreign ministry on Sunday condemned what it called the "violation of Ukraine's sovereignty and territorial integrity" by IAEA staff in its latest rotation.

Kyiv said the move was a "result of Russian blackmail and systematic attempts to impose illegal and contradictory operational mechanisms on international organisations in the temporarily occupied territories."

But Grossi told reporters in Vienna Monday that "the only thing that has led us... to do the rotation in this way is that I am responsible for the security of my staff.

"There were a series of episodes which led me to the conclusion that I could not take risks with the lives of my experts," he said, referring to a drone strike that affected a rotation in December as well as another mission that had to be aborted.

"Any other political consideration, I think, is out of place really."

Grossi has repeatedly warned of the potential for a nuclear accident at the plant.

According to Ukraine's foreign ministry, fighting in the region had prevented IAEA staff on site from being replaced "for 80 days", with Kyiv and Moscow blaming each other.

On Saturday, the Russian head of the Zaporizhzhia plant announced that an IAEA team had arrived at the facility for the first time via "Russian territory".