French nuclear energy firm Areva, whose Cogema unit will recycle the plutonium, said the shipment left Charleston, South Carolina, on Monday for France aboard two British ships.

"The cargo is protected by armed guards throughout its journey and the ships are equipped with naval guns," Areva said in a statement.

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.

The U.S. government said Areva had a permit to transport 308 lb of plutonium but the actual shipment was 275 lb. It said the program would remove weapons-grade plutonium from the grasp of terror groups.

"We're not just doing this for the heck of it. We're doing it for nonproliferation and security reasons," said Bryan Wilkes, spokesman for the U.S. National Nuclear Security Administration, an independent arm of the U.S. Energy Department.

"It's toward a non-proliferation goal of reducing the number of nuclear weapons and ensuring that this plutonium can never be used in a nuclear weapon again," Wilkes said.

Greenpeace criticized the transport, saying upon arrival in the northern French city of Cherbourg, the plutonium would be driven over 660 miles in highly vulnerable trucks to a factory in south-eastern France.

"This shipment sends the strongest signal that the United States holds little regard for global efforts to keep nuclear weapons materials out of commerce," Tom Clements of Greenpeace International said in a statement.

"It is the height of arrogance to conduct a shipment like this while demanding other nations refrain from proliferating nuclear weapons materials and technologies," he added.

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

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

The MOX plutonium-uranium enriched fuel is controversial because critics fear it could potentially be used to build nuclear weapons. (additional reporting by Chris Baltimore in Washington)