"What's going to be transported on French roads is the purest form of nuclear material for military use," a spokesman for the association, Tom Clements, told a Paris media conference.

The shipment of 140 kilogrammes (308 pounds) of plutonium from US weapons arsenals left the North Carolina port of Charleston on September 20 and is due to arrive under British escort "probably Friday" at the French port of Cherbourg, he said.

The nuclear matter is to be taken to the French nuclear reconditioning station at La Hague nearby, then sent on to a facility in southern France to be transformed into mixed-oxide, or MOX, fuel for civilian nuclear reactors and eventually returned to the United States.

Greenpeace said the long distances of road transport involved constituted "considerable" risk, not least because the cargo's containers could easily be opened by shoulder-launched rockets.

Cogema, the French state nuclear company, rejected the accusations in a statement, saying the transport of plutonium is carried out with "all safety guarantees" and the truck convoy would be unmarked to avoid attracting attention.

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