TOKYO (Reuters) - Japan pledged to reduce its controversial stocks of plutonium, the world’s biggest inventory of the highly toxic material held by a state without nuclear weapons, following pressure from the United States, China and other countries.

The government did not outline by how much and when it will cut the stockpiles of plutonium it holds. Japan shut down most of the reactors that can use the material as fuel after the Fukushima nuclear disaster of 2011.

Japan, the only country to be bombed by a nuclear weapon, is also the only nation without atomic arms to have significant amounts of plutonium. This has sparked criticism from nuclear-armed China and other neighboring countries, who are worried about the material falling into the wrong hands or that Japan might eventually build a weapon.

Japan held 47 tonnes of plutonium at the end of 2017, including 21 tonnes stored in Britain and 15 tonnes in France, enough to make thousands of atomic bombs.

Local media reported in June that the U.S. government had asked Japan to cut its stockpiles ahead of an extension this month of a nuclear cooperation agreement that allows the country to reprocess spent nuclear fuel and extract plutonium for further use in reactors.

“Japan will reduce the size of its plutonium stockpile,” the Japan Atomic Energy Commission (JAECO) said on Tuesday in a revision of the country’s basic policy on plutonium use that was first issued in 2003.

Plutonium is the foundation of resource-poor Japan’s nuclear fuel cycle, in which spent fuel from nuclear reactors is reprocessed into plutonium and then reused. Most of the reprocessing is done in France but Japan is building its own site to produce plutonium for reactor fuel at Rokkasho in northern Honshu.

Rokkasho has been hit with repeated delays and cost overruns and is now scheduled to start operations in the early 2020s. Once it does it will add to the plutonium stockpiles unless the fuel produced can be used in reactors.

“We are struggling to reduce the stockpile, so why do we need to produce more plutonium?” said Tatsujiro Suzuki, director of the Research Centre for Nuclear Weapons Abolition at Nagasaki University and a former vice-chairman of JAECO.

He said JAECO’s statement of intent was a “good first step ... (but) now is a good time to rethink the whole recycling program.”

Only a limited number of reactors in Japan can use MOX, a blend of uranium and the plutonium recycled from spent nuclear fuel. Four out of six reactors currently operating can use MOX and consume about 2 tonnes of plutonium a year.

Reactors need a special license to be able to use MOX fuel and experts say it is an expensive method of fuelling reactors compared to using enriched uranium.

Japanese utilities also have large amounts of spent fuel held in reactor buildings, a source of concern after the Fukushima disaster.