France ordered a thorough search of all its nuclear plants after environmental group Greenpeace said its activists were inside sites other than one outside Paris where nine were arrested Monday.

But French energy firm EDF, which runs the country's nuclear stations, said there were no signs that activists had managed to infiltrate other installations.

"There are no traces of intrusion in EDF's other nuclear stations," it said in a statement.

An interior ministry spokesman said the search was ordered after the break-in at the Nogent-sur-Seine plant near Paris but insisted that "at no point was there a threat to the integrity of nuclear installations."

"We are proceeding with an extensive search of all nuclear plants and installations," ministry spokesman Pierre-Henry Brandet told AFP.

Brandet said that the last two of nine activists who had entered the nuclear site at Nogent-sur-Seine, 95 kilometres (60 miles) southeast of Paris, had been detained.

"There were nine at Nogent, the last two have just been apprehended," he said.

Greenpeace said its activists had broken into other French nuclear sites after they entered the Nogent-sur-Seine plant early Monday to stage a protest.

"At around 6:00 this morning at the same time as they entered the Nogent site, other Greenpeace militants entered other nuclear installations," the group said, declining to name the sites in question.

Greenpeace nuclear expert Sophia Majnoni said it had taken only 15 minutes for the activists to reach the core of the Nogent-sur-Seine facility.

"In about 15 minutes the activists reached the heart of the plant, where the nuclear core and nuclear fuel are," she told journalists near the plant.

The activists "crossed four security barriers made of electric fences and barbed wire. They encountered no one" during their passage, she said.

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