China on Monday urged Japan to return over 300 kilograms of nuclear material, which could arm dozens of nuclear weapons, to suppliers and balance its nuclear stock.

"Japan's large stock of nuclear material, including weapons-grade nuclear substances, involves nuclear security and nonproliferation risks," foreign ministry spokeswoman Hua Chunying told a regular press briefing.

It is also against the regulations of the nuclear watchdog, the International Atomic Energy Agency (IAEA), which requires countries to keep a balance between the demand and supply of nuclear materials, Hua said.

The US offered 331 kilograms of weapons-grade plutonium, partly produced by Britain, to Japan during the Cold War, Japan's Kyodo News reported in late January.

The material was bought for research purposes, an official at Japan's Education Ministry said.

The highly concentrated plutonium, which is kept by Japan's atomic energy agency, could be used to produce 40 to 50 nuclear weapons. Japan holds another 44 tons of plutonium which could be used for nuclear reactors.

The US has been asking Japan to return the material since the first nuclear security summit in 2010. The two sides are expected to reach an agreement at the third summit in the Netherlands this March.

"Japan has avoided returning the material which caused international concern. China is deeply concerned and is expecting an explanation," Hua said.

She urged Japan, a member of the Treaty on the Nonproliferation of Nuclear Weapons, to honor its obligations and return the material at an early date.

"We also urge Japan to settle the imbalance between its need for and supply of nuclear materials in accordance with IAEA requirements," Hua said.