The FSB, successor to the Soviet-era KGB, said the diplomat was caught "red-handed while attempting to obtain secret information," a spokesman for the FSB said by telephone.

The FSB identified the detainee as Gabriel Grecu, saying he was employed by the intelligence services of NATO-member Romania "working undercover as the first secretary of the political department of the Romanian embassy" in Moscow.

Romania's foreign ministry declined immediate comment. The Romanian embassy in Moscow could not be reached for comment.

The FSB said in a statement that Grecu was caught with "spy equipment," without going into detail. Grecu worked at the Romanian embassy in Armenia in 2006, according to Romanian media.

Russia in February expressed concern over US plans to install interceptor missiles in Romania in what Washington said was defense against potential threats from Iran.

US missile shield plans have angered eastern Europe's former Cold War master Russia, who sees it as a threat to its own nuclear arsenal and has bristled at what it says is meddling by Washington in its sphere of influence.

Russia and Western nations accuse each other of significant amounts of spying, nearly two decades after the end of the Cold War.

Monday's detention follows the US arrests in June of 10 Russian undercover agents, who were later swapped for four Russians imprisoned for allegedly trading secrets with the West.