The Japanese government has not declared about 640 kg of unused plutonium in its annual report for the International Atomic Energy Agency in 2012 and 2013, an amount large enough to make 80 nuclear bombs, local media reported on Saturday.

Japan claims to own 44 tons of plutonium, while the actual amount is 45 tons, said Japan's Kyodo News Agency. The unreported plutonium is part of the plutonium-uranium mixed oxide fuel placed at an offline reactor in a nuclear plant in Saga Prefecture, southern Japan.

The MOX fuel was loaded in March 2011, shortly before the Fukushima nuclear crisis later that month. For the next two years, the unused fuel was removed from the reactor, which remained offline.

An official from the Japan Atomic Energy Commission argued that the plutonium is considered being used and hence exempt from reporting to the IAEA.

But plenty of experts at home and abroad criticized Japan's government for not recognizing the seriousness of the problem.

"From the safeguards point of view, this material is still unirradiated, fresh MOX fuel regardless of its location," former IAEA Deputy Director-General Olli Heinonen said. Thus, the unreported plutonium could be diverted to nuclear bombs.

Japan has the largest amount of plutonium among non-nuclear nations. The country used it for power generation in the past, but after the 2011 disaster at Fukushima, Japan's nuclear reactors remain idle.

The large amount of plutonium has prompted regional worries about Japan's motives, as well as global concerns over the security of these nuclear fuel reserves.