STOCKHOLM - The United States and the International Atomic Energy Agency (IAEA) said Wednesday that a floating cemetery of nuclear submarines off Russia could be a target for terrorists seeking parts for nuclear bombs.

Moscow played down the risks, saying it had stepped up security around the submarines off the northwestern Kola peninsula. And it said the nuclear waste could be cleaned up within a decade with about $200 million.

``Of course it's possible that a terrorist could make a 'dirty nuclear bomb' from the nuclear fuel on board the submarines,'' Michael Bell, head of the IAEA's waste technology section, told Reuters by telephone from a conference in the southwestern Swedish city of Oskarshamn.

Dieter Rudolph from the Defense Department, who was also attending the conference, agreed there were risks but said they were small. ``In theory it is possible but it would be a tough and heavy task to handle the radioactive fuel,'' he said.

The three-day conference, ending Thursday, is about Russia's problems with treating nuclear fuel waste and missiles aboard a fleet of some 150 disused submarines around the Kola peninsula.

Rudolph said that there were easier ways to find nuclear material to build a 'dirty bomb' from radioactive material. Such a crude bomb could cause serious damage although not as extensive as a properly built atomic bomb.

Last week, the IAEA warned the world that the threat of attacks on nuclear power plants had increased and urged countries with such stations to boost security.

It said the risk of airplane attacks and theft of nuclear material had increased in the aftermath of the Sept. 11 suicide hijacker attacks on the United States.

``Russia has taken emergency security measures because we know there is a real threat from international terrorism,'' Russia's deputy Atomic Energy Minister Valery Lebedev told Reuters.

He also played down fears of leaks from the aging submarines.

``At the moment there's not much leakage going on. What we are looking for is help to handle and reprocess the solid radioactive waste and spent fuel from the atomic submarines,'' he said.

``It will take about 10 years and cost about $200 million to remove and secure the waste,'' he said.

Rudolph said that Washington was most concerned about the nuclear missiles aboard the submarines.

``The U.S. focus is to pay the Russians to dismantle weapons of mass destruction but also to help remove, store and reprocess the radioactive nuclear fuel on the peninsula,'' he said.

The United States is planning to assist Russia in improving the capacity at a nuclear fuel reprocessing plant in the city of Mayak and to clean up buildings around the plant which are believed to be radioactive.

It will also assist in removing weapons and fuel from several submarines at a former military base in Andrejeva Bay.

But Rudolph and Bell agreed that although the submarines posed a threat to the environment should they sink and leak, the worst case scenario could never compare with that of the Chernobyl nuclear accident in 1986.