VIENNA (REUTERS) - The United States and China disagreed over Japan's plutonium stocks at a UN nuclear agency meeting on Wednesday, with Washington saying it did not share Beijing's concern about the sensitive issue, diplomats said.

China expressed concern about the size of Japan's plutonium holdings at a board session of the International Atomic Energy Agency (IAEA), diplomats who attended closed-door discussions at the UN body said.

Russia voiced similar views, they said.

Like uranium, plutonium can be used to fuel nuclear power plants, but can also provide material for nuclear bombs.

The US ambassador to the IAEA made clear his country was not worried about Japan's treatment of the material.

"We are not at all concerned that the plutonium is either being handled improperly or that there isn't a plan for disposition," Ambassador Joseph Macmanus told reporters.

He later told the board, according to one diplomat, that "we do not share the concerns expressed" by China last month.

On Feb. 17, Beijing said it was "extremely concerned" by a report that Japan has resisted returning to the United States more than 300 kg of mostly weapons-grade plutonium.

Japan's Kyodo news agency said the United States had pressed Japan to give back the nuclear material, which could be used to make up to 50 nuclear bombs. Japan had balked, but finally given in to US demands, Kyodo said.

The material was bought for research purposes during the 1960s and the two governments will probably reach an official agreement on its return at the Nuclear Security Summit in The Hague in March, an official at Japan's Education Ministry said.

Nuclear-armed China is involved in a bitter territorial dispute with Japan. It denies Japanese accusations that it is a threat to peace and in turn has accused Japan of trying to rearm and failing to learn the lessons of its brutal behaviour during World War Two, when Japanese forces occupied China.

Japan, the world's only target of atomic bombs, in the final stages of World War Two, does not have nuclear weapons, and says it will not seek to obtain them.

Japan has plutonium contained in spent nuclear fuel at civil reactor and reprocessing sites, totalling 159 tonnes at the end of 2012, according to Japanese data posted on the IAEA website.

Mr Macmanus said "plutonium and the disposition of plutonium stocks" was a central element of what he called a very successful diplomatic and energy partnership with Japan.

"We are satisfied that Japan understands what the conditions are for the use and the maintenance of those stocks and we are not concerned," he told reporters.

In his statement to the board, he was quoted as saying that one goal of a US-Japan nuclear security working group was to reduce quantities of weapons-usable nuclear material in Japan, and that this cooperation has been "successfully ongoing for decades". He said Japan had been "consistently" transparent about its plutonium inventory.

IAEA Director General Yukiya Amano earlier this week also said there was no reason for concern that plutonium held by Japan could be diverted for nuclear arms purposes.