A ship carrying up to 331kg of weapons-grade plutonium has left the port in Ibaraki Prefecture in Japan for the US, where the material will be downgraded. The amount of plutonium carried by the vessel is enough to produce up to 50 nuclear bombs.

The nuclear material is being carried by an armed British vessel, operated by Pacific Nuclear Transport, and its exact route has been kept secret for security reasons. The purity of plutonium is so high that it could be easily used to make advanced nuclear weapons.

Two vessels, equipped with naval guns and other sophisticated protection, flying British flags had arrived at Japan's coastal village of Tokai earlier. The transfer of plutonium onto the vessel itself takes several hours, reported Japan's state-backed broadcaster NHK news. The shipped plutonium consignment was mostly bought from the UK and some from the US and France. The ultimate destination of the cargo is the Savannah River Site in South Carolina.

The shipment — as part of the counter-terrorism deal between Japan and the US agreed in 2014 during a nuclear summit — is aimed at easing concerns over Japan's vast stockpile of plutonium. Japan possesses about 47 tonnes of plutonium — enough to make as many as 6,000 atomic bombs — located both inside and outside the country, apart from the latest shipment.

Tokyo purchased plutonium from the West in the 1970s for nuclear research and further created the material by reprocessing spent fuel from power facilities. However, except for two nuclear plants, all such reactors have been shut down after the 2011 Fukushima nuclear disaster. Neither Japanese officials nor the US embassy in Tokyo has officially commented on the shipping due to the sensitivity surrounding the matter.

The US environmental group Savannah River Site Watch has expressed concerns on why such materials need to be brought to America. The group's director Tom Clemens urged the US authorities to "reassess its position at the summit and push hard for Japan to cease reprocessing and plutonium stockpiling due to the proliferation threat those programs pose".