TOKYO (AFP) - An armed nuclear transport vessel preparing to ship a huge cache of plutonium - enough to produce 50 nuclear bombs - to the US arrived at a Japanese port on Monday (March 21) amid tight security, local media said.

The stockpile, provided by the US, Britain and France decades ago for research purposes, is being returned to the US as part of a bilateral storage deal.

Television footage showed the Britain-registered ship entering a port in Tokaimura, northeast of Tokyo, close to where the highly-toxic substance has been stored.

Tokyo Broadcasting System (TBS) reported that police boosted security around the port where anti-nuclear activists were also boarding a boat to monitor the arrival offshore. So far there have been no reports of protests.

TBS and Kyodo News said the ship is set to leave the port soon carrying the 331 kilogrammes (730 pounds) of plutonium, guarded by another vessel.

Comments from Japanese authorities on its departure or itinerary were not available, but anti-nuclear campaign groups said it will take two months for the ship to arrive at a nuclear facility in South Carolina.

A Japanese official told AFP last week that the material will be disposed of in the United States.

The stockpile 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.

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

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

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.