Dozens of activists have since Saturday awaited the arrival of the two ships, escorted by armed commandos, at the port of Cherbourg in northern France.

Activists question the wisdom of transporting such security-sensitive cargo at a time of heightened risk of terror attacks globally.

French nuclear energy firm Areva, whose Cogema unit will recycle the 140 kg (308 lb) of plutonium, said it would only release information on the shipment from Charleston, South Carolina, a few hours ahead of its arrival.

"The two ships transporting the plutonium should arrive yesterday night," said a Greenpeace spokesman. He said the activists were detained while putting up a protest banner.

The shipment is part of a post-Cold War agreement between the United States and Russia to get rid of plutonium from excess nuclear warheads.

Greenpeace criticised the transport, saying on arrival in Cherbourg the plutonium would be driven over 1,000 km (660 miles) in vulnerable trucks to a factory in southeast France.

Areva's Cogema unit will recycle the plutonium into nuclear fuel at its Cadarache and Marcoule plants in southeastern France and ship it back to the United States, which plans to use it in an electricity-generating reactor.

It is part of the U.S. Department of Energy's controversial programme to turn plutonium from the "excess" nuclear warheads into mixed-oxide (MOX) plutonium-uranium enriched fuel.

Critics fear the fuel could potentially be used to build nuclear weapons.