Electricity is essential to operate pumps that circulate water to cool reactors and pools holding nuclear fuel.

"Each time we are rolling a dice," IAEA chief Rafael Grossi told the agency's board of governors.

"And if we allow this to continue time after time then one day our luck will run out."

Grossi has been in consultations with Kyiv and Moscow for several months to try to set up a protection zone around the plant, but the talks appear to have stalled.

"We must commit to protect the safety and security of the plant," Grossi said. "And we need to commit now. What we need is action.

"Let me remind you - this is the largest nuclear power station in Europe," he said.

Grossi noted this was the sixth time that the Zaporizhzhia facility had been cut off from the electricity grid since Russia captured it a year ago, and the first time since November.

"This cannot go on," he said, adding, "I am astonished by the complacency -- what are we doing to prevent this happening? he asked, vowing to pursue his efforts.

The strike came during a fresh wave of Russian attacks across Ukraine which killed at least nine people and led to power outages across the country.

"The last power line between the occupied Zaporizhzhia NPP and the Ukrainian power system was cut off as a result of rocket attacks," the operator Energoatom said.

The emergency diesel generators can provide the facility's energy needs for 10 days, it said.

"The countdown has begun. If it is impossible to renew the external power supply of the station during this time, an accident with radiation consequences for the whole world may occur," Energoatom said.

Russian authorities that control the plant said diesel generators had been switched on following a "short-circuit" on power lines, without providing details.