Greenpeace activists secretly entered a French nuclear site before dawn and draped a banner reading "Coucou" and "Facile", (meaning "Hey" and "Easy") on its reactor containment building, to expose the vulnerability of atomic sites in the country.

Police, whom the environmental activist group immediately told of the publicity stunt, took several hours to round up nine intruders who had broken into the power plant in Nogent-sur-Seine, about 95km southeast of Paris, on Monday.

Greenpeace said the break-in aimed to show that an ongoing review of safety measures, ordered by French authorities after a tsunami ravaged Japan's Fukushima Dai-ichi nuclear plant earlier this year, was focused too narrowly on possible natural disasters, and not human factors.

"With this nonviolent action, Greenpeace has shown how vulnerable French nuclear plants are," said Sophia Majnoni d'Intignano, a Greenpeace activist.

Activists who tried to enter three other French nuclear sites, in a co-ordinated action on the same day, were prevented from doing so, but Greenpeace said other invaders were still holed up inside other, unspecified, nuclear sites.

'Thorough sweep'

Authorities had launched a "thorough sweep" of all of France's 20 nuclear power plants following Monday's incursion, Pierre-Henry Brandet, the interior ministry spokesman said.

"We have to understand what's behind this malfunction, notably in Nogent," Brandet said, adding that "in the other sites security worked ... the intrusions were thwarted".

French power company Electricite de France (EDF), which operates the site, denounced the "illegal" break-in at Nogent-sur-Seine and insisted that it did not harm security at the site.

EDF said activists' banners were also hung on the outside of two other nuclear sites - Chinon in northwestern France and Blayais in the southwest - before they were removed.

Three other activists were driven off by security forces while trying to enter yet another plant, in southeastern Cadarache.

EDF said it had no indication of intrusions at other sites in France.

'Risk of terrorism'

D'Intignano said the government is going to conclude in several months that French nuclear plants are safe, because it is believed that they could withstand a flood or an earthquake.

"But those aren't the real risks for our nuclear industry," Majnoni d'Intignano said. "It's the risk of [an] external, non-natural attack, like the risk of terrorism."

Speaking later by phone with the AP news agency, she urged the government to consider other risks in its review like an airplane crash, a computer virus, or chemicals explosion at a nuclear site.

"It's a very limited review," she said. "For us, the real risks are human and technological."

Henri Guaino, a special adviser to French President Nicolas Sarkozy, called the intrusion "irresponsible," but said, "I think we'll have to learn some lessons."

France, which gets about three-quarters of its electricity from nuclear power, regularly faces protests from environmental activists over shipments of nuclear waste. But, activist incursions into atomic plants are unusual.

Claude Gueant, the interior minister, has scheduled a meeting this week to launch a review of the security breach, Brandet said.