Greenpeace activists managed to sneak into a French nuclear power plant on Monday, police have confirmed.

In a statement Greenpeace said some members had entered the nuclear site at Nogent-sur-Seine, 95 kilometres (60 miles) southeast of Paris, to "spread the message that there is no such thing as safe nuclear power."

Eight activists entered the power station site according to a source with the French police, who added that some of the intruders had already been apprehended.

"A group of militants managed to climb onto the dome of one of the reactors, where they unfurled a banner saying 'Safe Nuclear Power Doesn't Exist'," said Greenpeace spokesman Axel Renaudin.

"The aim is to show the vulnerability of French nuclear installations, and how easy it is to get to the heart of a reactor," said Sophia Majnoni, a Greenpeace nuclear expert.

She denounced a government security audit of its nuclear plants as "a communications exercise which does not take into account risks already identified in the past and does not learn the lessons of Fukushima," the Japanese nuclear plant which was crippled by an earthquake and tsunami in March 2011.

The Nogent-sur-Seine plant, run by the EDF energy company, was chosen by Greenpeace "because it is the nearest to Paris," Greenpeace said.