The UN nuclear watchdog says one of the reactors at the Zaporizhzhia nuclear power plant in Ukraine has been shut down again.

The International Atomic Energy Agency announced on Saturday that the No.5 reactor -- one of two operational reactors at the Russian-controlled plant -- was disconnected.

The same reactor was forced to shut down on Thursday after nearby shelling activated a safety protocol. It resumed operation the following day.

The agency said the No.6 reactor is still operating and producing electricity for cooling, household use and other purposes.

IAEA experts also quoted senior Ukrainian staff at the facility as saying the plant's last remaining external power line was down. Three others had been lost earlier in the conflict.

But the experts said a reserve line linking the facility to a nearby thermal power plant was delivering the electricity the plant generates to the external grid.

Ukraine and Russia each blamed the other for the issues. Ukraine's state nuclear energy firm, Energoatom, claims the No.5 unit was shut down due to Russian shelling. But Russia maintains that Ukraine is responsible.

A team of IAEA experts started inspecting the plant on Thursday to ensure its safety. Lingering safety concerns make it uncertain whether they will be able to stay there and continue their monitoring activities.