John Rockefeller, vice chairman of the Senate intelligence committee, said loose Russian radioactive materials and possibly other components of nuclear weapons made him wonder whether Russia was a greater proliferation threat than North Korea, which has publicly claimed to have a nuclear weapons arsenal.

Members of the committee received last week a detailed classified briefing from Central Intelligence Agency officials on security threats the United States faces.

"In the sense that half of the nuclear materials, pieces and parts of it, are unaccounted for by the Russians -- and a lot of them, these places are in rural areas -- I think you can ... have a real debate as to which is more threatening to the world right now," said the Democratic senator, appearing on the "Fox News Sunday" television program.

The warning came as US President George W. Bush arrived in Europe as part of a fence-mending visit aimed at smoothing over disagreements with allies on the Iraq war and strengthening transatlantic security cooperation, including in nuclear non-proliferation.

Bush is scheduled to discuss this and other issues with his Russian counterpart, Vladimir Putin, when the two meet in the Slovakian capital of Bratislava on Thursday.

Rockefeller questioned Russia's ability to guard its nuclear arsenal due to widespread corruption in the country.

He insisted that "a lot of those lost nuclear weapons can be out circulating in the terrorist community" and president Putin "ought to be very worried" that these weapons or materials could end up in the hands of Chechen separatists.

The CIA, which monitors proliferation of weapons of mass destruction, declined to comment on the senator's remarks.

Russia has thousands of nuclear warheads and dozens on nuclear production and research facilities handling fissile substances. It was not immediately clear which materials the senator referred to. Calls to his office with requests for clarification remained unanswered on Sunday.

During a Senate hearing last week, Rockefeller read from a yet-to-be publicly released report by the National Intelligence Council, a CIA think tank that analyzed security arrangements at Russian nuclear sites.

Russian officials, the senator quoted the report as saying, have acknowledged that extremist groups have already targeted Russian nuclear weapons storage sites.

Two known attempts by terrorists to reconnoiter such facilities were thwarted in Russia prior to 2001, incidents that prompted Moscow to heighten security throughout its nuclear complex, according to the senator.

"We find it highly unlikely that Russian authorities would have been able to recover all the material reportedly stolen," he read from the report. "We assess that undetected smuggling has occurred, and we are concerned about the total amount of material that could be diverted or stolen in the last 13 years."

CIA Director Porter Goss declined to discuss in a public hearing whether the missing material would be sufficient to build a nuclear weapon.

However, when asked if he could assure the American people that nuclear material from Russia had not found its way into terrorist hands, he replied: "No, I can't make that assurance. I can't account for some of the material, so I can't make the assurance about its whereabouts."

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