A large shipment of plutonium is expected to depart Japan soon amid a warning from a senior American official saying nuclear reprocessing in East Asia could lead to increased amounts of nuclear material that could be used for nuclear weapons.

By late Sunday, two armed British transport ships currently docked in Kobe, the Pacific Egret and the Pacific Heron, are to be dispatched to the Japan Atomic Energy Agency's port in the village of Tokai, Ibaraki Prefecture, according to Greenpeace, which is monitoring the ships.

The vessels will pick up 331 kg (729 pounds) of plutonium that was sent to Japan by the United States for civil research years ago but can also be used for nuclear weapons. The material will be returned to the U.S. Department of Energy's Savannah River Site in South Carolina in a trip expected to take about two months.