A nuclear transport vessel carrying a huge cache of plutonium -- enough to produce 50 nuclear bombs -- to be returned to the US left a Japanese port on Tuesday afternoon.

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

Television footage by national broadcaster NHK showed containers being loaded on to the Britain-registered ship at a port in Tokaimura, northeast of Tokyo, close to where the highly-toxic substance has been stored, and the ship leaving the port.

The vessel carrying the 331 kilogrammes (730 pounds) of plutonium departed at about 3 pm (0600 GMT) under tight security guarded by patrol boats of the Japan Coast Guard, according to NHK.

The government "cannot reveal" the vessel's itinerary or any other details, an official told AFP, 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 US.

Five anti-nuclear groups, including Greenpeace, said in a joint statement on Tuesday released ahead of the vessel's departure that the shipment "exposes the failure of global plutonium reprocessing programmes and the threat from current Japanese nuclear policy."

The stockpile has caused disquiet among anti-nuclear campaigners 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 including Greenpeace last week 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.

In Beijing, China's foreign ministry on Tuesday called on Japan to take further steps to reduce its stockpiles.

"In addition, it should return large quantities of other materials, including weapons-grade plutonium and enriched uranium," ministry spokeswoman Hua Chunying told reporters.

"This is indeed a concern of the international community."

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.