Operations at the sixth and final reactor at the Zaporizhzhia nuclear power station in Ukraine's south were shut down, the country said Sunday.

"Today, Sept. 11, 2022, at night, at 3:41 a.m. (1:41 a.m. GMT), unit No. 6 of the ZNPP was disconnected from the power grid. Preparations are underway for its cooling and transfer to a cold state," state nuclear agency Energoatom said in a statement.

Ukraine and its allies have been increasingly concerned about the safe operation of the Zaporizhzhia plant — the largest in Europe — and recent fighting there has raised fears of a serious incident.

The U.N.'s atomic watchdog warned earlier this week that a blackout in the nearby town of Energodar had "compromised the safe operation" of the nuclear facility.

Energoatom said Sunday that a cold shutdown was the "safest state" for the reactor.

Energoatom said that the sixth reactor had been generating energy for the plant itself for three days and that the decision to halt its operations came when external power had been restored to the facility.

"In case of repeated damage to the transmission lines linking the facility to the power system — the risk of which remains high — the (plant's) in-house needs will be powered by diesel generators," it cautioned in a statement.

Energoatom, in its statement, again called for establishing a demilitarized zone around the plant, saying it was the only way to ensure the plant's safety.

Under Russian control since March, the plant in southeastern Ukraine was disconnected from the country's power grid last Monday, amid growing concerns of a nuclear disaster as Moscow and Kyiv accuse each other of attacks on the nuclear facility.

Energoatom said a power line was restored on Saturday to allow officials to "shut down power unit No. 6 and transfer it to the safest state - cold shutdown."

The agency warned that the risk of more damage and outside power being cut again "remains high."

In such a scenario, the plant will have to be "powered by diesel generators, the duration of which is limited by the technical resource and the amount of available diesel fuel," the statement added.

Zaporizhzhia, one of the 10 largest nuclear power plants in the world, generated 20% of Ukraine's electricity before the war.

A team of experts from the International Atomic Energy Agency (IAEA) led by its chief Rafael Grossi inspected the facility last week.

Briefing the U.N. Security Council on his report, Grossi said the damage wreaked by attacks on the facility was "simply unacceptable," warning that it was like "playing with fire" and could lead to "something very catastrophic."

The Energoatom statement also called on Russian forces to withdraw from the area and "create a demilitarized zone around it."