Russia's Federal Security Bureau said today it had uncovered a US plan to steal Russian military secrets, Interfax news agency reported.
CIA officials posing as US diplomats tried to recruit an expert in a secret Defence Ministry plant before the FSB intervened, the agency said. It said Russia's national security had not been jeopardised.
"The FSB has irrefutable evidence of the CIA's spying activities in Russia," an FSB official was quoted as saying.
"The work was carried out by CIA officers, working under the cover of American diplomats in Moscow and in one of the CIS states," the unnamed official said.
He named a junior diplomat in the US Embassy in Moscow as leading the operation, adding the diplomat had already left Russia.
The US Embassy declined to comment on the accusations, which come just over a month before Russian President Mr Vladimir Putin and US President Mr George W Bush are due to meet in Moscow and St Petersburg.
The FSB, the main successor to the Soviet-era KGB, was unavailable for comment.
In March 2001, 50 Russian diplomats were expelled from the United States, prompting a tit-for-tat response from the Kremlin in the worst spy scandal to shake Moscow and Washington since the Cold War.