Electrabel temporarily closed a nuclear reactor on Sunday after an electrical fire, Belgium's electricity transmission system operator said, leaving only three of the Belgian firm's seven nuclear plants in action.

Electrabel, part of France's GDF Suez, had to halt the 1,000-megawatt Tihange 3, southwest of the city of Liege, after several electrical cables outside the reactor caught fire.

Electrabel was not immediately available for comment. Belgian news agency Belga quoted a company spokesman as saying the cause of the fire was a technical fault.

"On Sunday morning, a fire happened at Electrabel's high-voltage power station on the site of Tihange 3," said a spokeswoman for Elia, Belgium's electricity transmission system operator. "Due to the fire, the generation unit was automatically disconnected from the grid."

It was not immediately clear when Tihange 3 would restart.

Electrabel operates seven nuclear reactors: four in Doel near the northern city of Antwerp and three in Tihange with a total capacity of almost 6,000 megawatts, covering about half of Belgium's electricity demand.

However, the ageing reactor network is facing serious problems. With two of the four reactors at Doel offline and another reactor at Tihange already out of action, energy experts have raised the spectre of possible blackouts this winter. (Reporting by Barbara Lewis; writing by Robin Emmott; editing by David Clarke)