Japan's key ally the US has been pressing the country to return more than 300 kg of mostly weapon-grade plutonium that it exported to Japan for research purposes during the Cold War era, media reported.

 

 The plutonium that is stored at a fast critical assembly in Tokaimura in Japan's Ibaraki prefecture could be used to produce 40-50 nuclear weapons, reported Japan's Kyodo News, citing unnamed Japanese and US government officials, according to Xinhua.

 

 Japan has strongly resisted the demand raised by US President Barack Obama's administration, but it finally gave in to repeated demands, Kyodo said.

 

 The two countries since last year have been seriously discussing the issue as the US plans to reach an accord with Japan at the third nuclear security summit in March in the Netherlands, according to the report.

 

 The fast critical assembly belonging to the Japan Atomic Energy Agency is the country's only critical assembly designed to study characteristics of fast reactors.

 

 The Japanese ministry of education, culture, sports, science and technology and other researchers have argued that the plutonium in question is needed for research and vital for producing good data, said Kyodo.

 

 At present, Japan has another estimated 44 tonnes of plutonium, but its quality is not on par with the plutonium used for research purposes, Kyodo quoted a Japanese expert as saying.