The Zaporizhzhia nuclear plant (ZNPP) -- Europe's largest atomic facility -- has been shelled in recent weeks, with Kyiv and Moscow blaming each other for the attacks, raising concerns of a possible incident.

"Power unit (reactor) No. 6 was unloaded and disconnected from the grid" because of a fire that was "triggered because of shelling", state-run company Energoatom said in a statement on Monday.

"The world is once again on the brink of a nuclear disaster. The de-occupation of the ZNPP and the creation of a demilitarised zone around it is the only way to ensure nuclear safety," Ukraine's Energy Minister German Galushchenko said following the news on Monday.

This was the last working reactor out of six, after shelling disconnected reactor number 5 on Saturday, according to a statement from the International Atomic Energy Agency (IAEA).

"After the ZNPP connection to its last remaining operational 750 kilovolt (kV) line was lost late on Friday, the 330 kV reserve line had been used to deliver electricity from the ZNPP to the grid," the UN nuclear agency said on Monday.

"Ukraine informed IAEA that this back-up line will be re-connected once the fire has been extinguished.

"A secure off-site power supply from the grid and back-up power supply systems are essential for ensuring nuclear safety," it added.

- 'Worrisome' -

EU high representative and vice president Josep Borrell said the news was "worrisome" at a press conference alongside Ukraine's Prime Minister Denys Shmygal in Brussels.

Borrell said the "nuclear gamble has to stop" and accused Russia of "reckless behaviour, disdain for international law, basic principles of nuclear safety".

Last week, a 14-strong team from the IAEA visited Zaporizhzhia, with the UN nuclear watchdog's chief Rafael Grossi saying the site had been damaged in fighting.

Grossi and part of his team left the site on Thursday, but several members of the mission stayed at the facility to conduct more in-depth analysis.

Out of the six experts that stayed in the facility, four left on Monday and the remaining two are expected to remain in the power plant "on a permanent basis", according to an Energoatom statement on Monday.

Grossi will on Tuesday issue a report about nuclear safety in Ukraine that will include the mission's findings, the IAEA said, and he will brief the United Nations Security Council on the same day about the visit.

Ukraine was the scene of the world's worst nuclear disaster in 1986, when a reactor at the northern Chernobyl plant exploded and spewed radiation into the atmosphere.