Washington has been pressing Tokyo to return over 300 kg of mostly weapons-grade plutonium given to Japan for research purposes during the Cold War era, Japanese and U.S. government sources said Sunday.

President Barack Obama’s administration, which is keen to ensure nuclear security, wants Japan to return the plutonium supplied for use as nuclear fuel in a fast critical assembly in Tokai, Ibaraki Prefecture, the sources said.

The highly concentrated plutonium could be used to produce 40 to 50 nuclear weapons.

Japan has strongly resisted returning the plutonium, which it says is needed for researching fast reactors. But it has finally given in to repeated U.S. demands, the sources said.

Since last year, Japan and the United States have been actively discussing the matter, and Washington plans to forge an accord with Tokyo on the occasion of the third nuclear security summit in March in the Netherlands.

To prevent nuclear materials from falling into the hands of terrorists, the U.S. government has called for eliminating and minimizing the use of such materials. Since the first such summit was held in 2010 in Washington at the initiative of Obama, the United States has been pressing Japan to return 331 kg of plutonium now kept at the Japan Atomic Energy Agency’s fast critical assembly, the sources said.

The facility, which attained criticality in 1967, is the nation’s only critical assembly designed to study the neutron characteristics of fast reactors.

Since some of the plutonium was made in Britain, the United States is also asking London’s permission to transfer all of it to the United States, the sources said, adding the three nations are working out their policies on the matter, the sources said.

The Education, Culture, Sports, Science and Technology Ministry and other researchers have argued that the plutonium in question is needed for research and is vital to producing good data.

At present, Japan has about another 44 tons of plutonium, but its quality is not on a par with the plutonium used for research purposes, a Japanese expert said.

Since the March 2011 nuclear crisis at Fukushima No. 1 nuclear power plant, the United States has expressed its concern to Tokyo over how it will use plutonium.