"At the end of 2003, there were more than 3,700 metric tons of plutonium and highly enriched uranium - uranium enriched to 20 percent or uranium-235 - enough for hundreds of thousands of nuclear weapons, in about 60 countries," Albright and Kimberly Kramer wrote in an article to be published in the next issue of the Bulletin of the Atomic Scientists.

Most of the weapons-useable material is in Russia, followed by the United States.

In response to intelligence reports that terrorists are interested in acquiring nuclear weapons, the United States and Russia are working with the U.N. International Atomic Energy Agency (IAEA) to recover and secure all U.S. and Russian bomb-grade material spread across the globe.

Other states with some plutonium or HEU include the other declared nuclear powers - Britain, France and China - as well as Belgium, Italy, Germany, Japan, Netherlands, Spain, Sweden, Switzerland and former nuclear power South Africa, ISIS says.

North Korea, which withdrew from the nuclear Non-Proliferation Treaty (NPT) last year, had some 15 to 39 kg of plutonium and two to nine nuclear weapons at the end of 2003, according to a table in the article.

The article says that military plutonium stocks are also growing in Israel, Pakistan and India - countries known to possess nuclear weapons but which have not signed the NPT and are therefore not subject to IAEA safeguards.

The fact that states outside the NPT continue to make bomb material highlights the need for "an international ban on the production of fissile material for nuclear weapons", it says.

Albright and Kramer are not optimistic: "Civil plutonium stocks are not expected to decrease in the next 15 years."

WORRIES ABOUT SECURITY

It takes around 10 kg of plutonium-239 or 16-25 kg of HEU enriched to around 90 percent uranium-235 (U-235) to fuel a weapon. ISIS estimates that at the end of 2003 there was a total of 1,855 tonnes of plutonium and 1,900 tonnes of HEU globally. Most of the plutonium was in civilian hands, while the HEU was mostly in military stocks.

Some of the weapons-useable nuclear material produced around the world is disposed of, but the total amount keeps growing, Albright and Kramer say in their article, an advance copy of which was provided to Reuters.

"This is worrisome not only because the world has yet to come up with an accepted method of plutonium disposition but also from a security standpoint - how safe is that plutonium and HEU?"

Coastal countries like Ireland, New Zealand and Peru complain about the security of transporting nuclear materials through their territorial waters. These countries say that dangerous shipments of fissile or highly radioactive materials are often moved through their waters without their knowledge.

The environmental pressure group Greenpeace says on its Web site that 140 kg of plutonium - enough for at least 14 weapons - is now en route to France, where it is to be converted into nuclear reactor MOX fuel.