Belgium stepped up controls on nuclear safety that found cracks in the casing of an ageing reactor and led to two sites now closing after learning of signs of fissures in French atomic power plants, the national watchdog said Thursday.

Controls carried out in 2004 on a site in southern France revealed "indications of fissures" in base material "perpendicular to the (reactor's) surface" which were "dangerous", the head of Belgium's federal agency for nuclear safety AFCN Willy de Roovere told media.

Belgium therefore decided to check for similar problems, only to find "something else," he said, after talks in Brussels gathering EU nuclear states plus Switzerland and the United States.

De Roovere said June and July tests had suggested there could be "thousands" of possible fissures inside a giant, multi-cylindrical structure that serves as a protective vessel holding the Doel 3 reactor, located 25 kilometres (20 miles) north of Antwerp, and that these probably dated back to its construction decades ago.

The Dutch firm behind that job is long since out of business, and repairs have been described as practically impossible.

He said as these were parallel there was little risk compared to perpendicular cracks, but that he felt "a malaise given the large number" of defects, hence the decision to close that and a second reactor in the country's south near Liege.

It would be "difficult to prove" the Doel 3 site is watertight safe, he said, suggesting its permanent closure was all but certain.