Russian authorities expressed skepticism Monday about a German government report that a small amount of weapons-grade plutonium had been smuggled from Russia to a local businessman's home in Stuttgart.


"There have been no confirmed instances of weapons-grade plutonium leaving Russia," said Alexei Kandairov, a spokesman for the Federal Counterintelligence Service. "Interpol has sent us a request on this matter, and investigation is underway."


In May, German police found six grams of plutonium -- a small fraction of what is needed to build a bomb -- in the home of a Stuttgart businessman. On Saturday, Germany's Chancellery Minister Bernd Schmidbauer said on television that authorities had traced the source of the materials to Russia. He charged that both international underworld groups and Russian officials were involved in the transfer.


"Perhaps the most dramatic thing about it is that this material came from Russia's nuclear weapons industry," Reuters quoted him saying.


If confirmed, the plutonium transfer would mark the first time officials have proved that weapons-grade nuclear material has been illegally smuggled from Russia for sale abroad.


Concern that Russia could become a major source of black-market nuclear weapons fuel has grown among foreign governments in recent months. Some officials say that such a transfer is increasingly likely because of the disorder in Russia and poor security at its nuclear stockpiles.


Criminal trade of Russian nuclear weapons and fuel is "the greatest long-term threat to the security of the United States," FBI director Louis Freeh said at a recent Senate hearing in Washington.


In June, Sergei Stepashin, the head of Russia's Counterintelligence Service, the former KGB, said he did not believe that Russia's nuclear facilities were at risk