Japan will send a huge cache of plutonium -- enough to produce 50 nuclear bombs -- to the United States as part of a deal to return the material that was used for research, reports and officials said Tuesday.

The plutonium stockpile, provided by the US, Britain and France decades ago, has caused some disquiet given that Japan has said it has the ability to produce a nuclear weapon even if it chooses not to.

Some 331 kilograms (730 pounds) of the highly fissionable material will be sent by ship to a nuclear facility in South Carolina by the end of March, Kyodo News reported Monday in a dispatch from Washington that cited unnamed Japanese government sources.

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

It will be one of Japan's most significant overseas movements of plutonium since it transported one tonne from France in 1993 to be used in nuclear reactor experiments.

That shipment triggered an outcry at the time from countries citing environmental and security concerns.

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

"But we can't comment on further details, including the departure date and route, for security reasons," the official in the nuclear technology section at the education ministry told AFP Tuesday.

The material has been stored at the Nuclear Science Research Institute northeast of Tokyo, he added.

Japan relies heavily on nuclear technology for its energy needs.

In 2006, then foreign minister Taro Aso sparked panic in neighbouring countries by saying Japan, a scientific superpower with numerous Nobel prizes to its credit, had the know-how to produce nuclear arms but opts not to.

Japan is the only country to ever have been attacked with nuclear weapons, and under a 1967 policy it refuses to produce, possess or allow nuclear weapons on its soil.

But in 2010 Tokyo admitted to previous secret agreements with the United States to allow American warships to carry nuclear weapons across Japanese territory and to take the arms to US bases on Okinawa island in an emergency.

US atomic bombs obliterated the Japanese cities of Hiroshima and Nagasaki in the closing days of World War II, killing more than 210,000 people.