TOKYO (AP) — An armed British ship believed to be carrying enough plutonium to make about 40 atomic bombs left a port in eastern Japan on Tuesday to bring the shipment to the U.S. for storage.

The ship, operated by Pacific Nuclear Transport Ltd., was to take the 331 kilograms (730 pounds) of plutonium to a U.S. government facility in South Carolina under Japan's 2014 pledge.

The British-flagged, armed nuclear fuel transport ship Pacific Egret left the port in Tokai village, northeast of Tokyo, one day after arriving with another armed ship that had waited off-shore, Kyodo News agency reported. Tokai is home to the Japan Atomic Energy Agency, a nuclear research complex where the plutonium had been used for research.

JAEA refused to confirm the shipping details, citing security reasons.

Japan's stockpile and its fuel-reprocessing ambitions to use plutonium as fuel for power generation have been a source of international security concerns.

Japan has accumulated a massive stockpile of plutonium — 11 metric tons in Japan and another 36 tons that have been reprocessed in Britain and France and are waiting to be returned to Japan — enough to make nearly 6,000 atomic bombs.