Igor Sutyagin, who works at the prestigious USA and Canada Institute in Moscow, is being investigated under Article 275 of the criminal code - high treason - which covers spying.

FSB officials confirmed the charge, but refused to give further details.

Mr Sutyagin heads the military and technical studies section at the research institute in Moscow and is an acknowledged expert on strategic nuclear weapons.

"They are investigating him under the suspicion of spying for a foreign state," the institute's director, Sergei Rogov, told Reuters news agency.

"It is a very serious matter. Of course, it is only the court which could decide whether he is guilty or not," he added.

A report in The Washington Post quoted unidentified sources as saying that Mr Sutyagin was suspected of spying for the United States.

Flats searched

Pavel Podvig, editor of a reference work on Russia's nuclear weapons, said Mr Sutyagin's flat had first been searched on 27 October and that he had been charged on 5 November by the FSB, one of the successors to the Soviet-era KGB security police.

"I'm 100% confident he didn't do anything wrong," Mr Podvig said.

Mr Sutyagin, a physics graduate, worked with Mr Podvig on the reference book and is listed as one of its contributors.

Mr Podvig said his own flat had been searched twice and the FSB confiscated all the remaining 500-600 copies of his book.

A US PhD student from Princeton University, Joshua Handler, was also questioned during the investigation and his Moscow flat was searched. He has since left Moscow.

A number of Russian military researchers have been investigated or charged in recent months in unrelated incidents.