BEIJING - China on Monday called on Japan to explain its underreporting of plutonium to the International Atomic Energy Agency (IAEA) and resolve its oversupply of nuclear materials.

"We expect Japan to respond to the concerns of the international community, take practical action at an early date and address the imbalance between its demand and supply of sensitive nuclear materials," Foreign Ministry spokeswoman Hua Chunying said at a daily press briefing.

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Hua's comments came after the media reported that the Japanese government has not declared about 640 kg of unused plutonium in its annual report for the IAEA in 2012 and 2013, an amount enough to make 80 nuclear bombs.

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 (MOX) fuel placed at an offline reactor in a nuclear plant in Saga Prefecture, southern Japan.

"It is both the IAEA's requirement and Japan's duty to report its storage and use of nuclear materials," Hua said.

"Only in this way can the IAEA's guarantee and supervision be meaningful," Hua said.

She said Japan's long-term storage of sensitive nuclear materials has outweighed Japan's needs and aroused the serious concern of the international community.