Europe faced the prospect of a radiation disaster on Thursday when a Russian-occupied nuclear power plant was disconnected from Ukraine's electricity grid, Ukrainian President Volodomyr Zelensky said.

Zelensky added that only because of the back-up electricity supply, the Zaporizhzhia plant was able to operate safely, Telegraph reports.

The fires had previously damaged power lines, shutting down the plant.

There is growing concern about fighting near the complex, which is Europe's largest nuclear power plant.

"If the diesel generators had not been turned on, if our automation and plant staff had not reacted after the power outage, then we would already be forced to overcome the consequences of the radiation accident," Zelensky warned Thursday night.

The damage was caused by fires that Ukraine's state nuclear agency said had interfered with power lines connecting the plant on Thursday, temporarily disconnecting Zaporizhzhia from the national grid for the first time in its history.

"As a result, the station's two functional power units were disconnected from the grid," Kyiv officials said.

Satellite images taken on Wednesday showed an extensive fire in the vicinity of the nuclear complex.

The president blamed Russian bombing for the damage and in his overnight speech accused Moscow of putting Ukraine and Europe "one step away" from disaster.