Japan will return a huge cache of plutonium -- enough to produce 50 nuclear bombs -- to the United States starting this weekend as part of a bilateral storage deal, Greenpeace and other anti-nuclear campaigners said Friday.

The stockpile, provided by the US, Britain and France decades ago for research purposes, has caused disquiet among anti-nuclear groups and neighbouring countries given Japan has the know-how to produce a nuclear weapon -- even as it adamantly chooses not to.

Some 331 kilogrammes (730 pounds) of plutonium will be sent by "an armed British nuclear transport ship... under armed escort to the United States," said a joint statement by five groups, including Greenpeace.

The departure from a port north of Tokyo "could occur as early as Sunday," Shaun Burnie, senior nuclear specialist at Greenpeace, told AFP by telephone.

The shipment, which comes ahead of a nuclear security summit in Washington from late this month, is meant to underscore both countries' commitment to nuclear non-proliferation and is part of a 2014 deal.

"The material will be disposed of in the United States," Tsukasa Yamamura, an official in the nuclear nonproliferation technology section at the Ministry of Education, Culture, Sports, Science and Technology, told AFP Friday, referring to the deal.

But the campaigners condemned the shipment as a "dangerous distraction" from what they said is a far larger cache of roughly 10 tonnes of plutonium in Japan.

"Hailing a shipment of hundreds of kilogrammes of plutonium as a triumph for nuclear security, while ignoring over nine tonnes of the weapons material stockpiled in Japan and in a region of rising tensions, is not just a failure of nuclear non-proliferation and security policy but a dangerous delusion," Burnie said.

A Japanese official in January confirmed the amount of plutonium to be sent to the US and said that preparations for the shipment were under way.

But Yamamura declined to elaborate.

"We can't comment on details such as the departure date and route for security reasons," he said.

It will take two months for the ship to arrive at a nuclear facility in South Carolina, the campaign groups said.

Japan traditionally has relied heavily on nuclear technology for its energy needs, though the aftermath of the March 2011 Fukushima nuclear disaster led to all of the country's reactors being shut down, with only a few coming back on line so far.

In 2006, then foreign minister Taro Aso sparked neighbouring countries' concern and anger after saying Japan, a scientific superpower with numerous Nobel prizes to its credit, could produce nuclear arms though opts not to.