VIENNA, March 5 -- China urged Japan to address concerns over Japan's weapons-grade nuclear material during a UN nuclear agency meeting Tuesday, a senior Chinese diplomat said.

China has great concerns about a report that Japan refuses to return to the United States its large stockpile of nuclear material, including 300 kg of weapons-grade plutonium and 200 kg weapons-grade uranium, said Cheng Jingye, China's envoy to the International Atomic Energy Agency (IAEA).

He urged Japan to clarify the issue and take substantive steps to address the concerns over the issue.

While most of the civilian-use nuclear plants only need relatively lower enriched or quality nuclear fuel, a large stockpile of weapons-grade uranium and plutonium could be used as the fuel of nuclear bombs.

The large stockpile of weapons-grade nuclear material has potential nuclear proliferation risks, though the material is under the agency's safeguard, Cheng said.

Cheng noted Japan's holding of a large amount of plutonium is obviously inconsistent with an IAEA document of Guidelines for the Management of Plutonium, which stresses the importance of balancing supply and demand of plutonium.

The Chinese envoy said that the IAEA should continue to monitor in a strict and effective manner those counties that do not have nuclear weapons but own a large quantity of nuclear material and facilities.