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.