Ukraine's Russian-occupied Zaporizhzhia nuclear plant again lost connection to its last remaining main external power line overnight and was switched to a reserve line, state-owned power generating company Energoatom said on Thursday.

Energoatom said Europe's largest nuclear power plant was on the verge of a blackout as the reserve line had less than half the power capacity of the main power line.

"Such a regime is difficult for the reactor plant, its duration is limited by the project's design and it can result in failure of the main equipment of the energy unit," Energoatom said on Telegram.

The Zaporizhzhia plant with its six reactors has been controlled by the Russian military since the early days of Moscow's invasion in February 2022.

It has become one of the focal points of the conflict, with both sides blaming each other for shelling around the site. The International Atomic Energy Agency (IAEA) has been trying to set up a safety mechanism to prevent accidents.

None of the plant's six reactors produce electricity.

Separately, the station's Russian-installed administration said the Number 4 reactor had been moved from a "hot" to a "cold" shutdown because of signs of a steam leak.

One of the six reactors needs to be in "hot shutdown" to produce steam for the plant's own needs.

"Plant personnel found signs of leaks in the pipe section of steam generator No. 3," the administration said on Telegram. "To meet the steam auxiliary needs of the Zaporizhzhia NPP, the transfer of power unit No. 6 to a 'hot shutdown' state is being considered." (Reporting by Olena Harmash; Editing by Angus MacSwan)