The Federal Security Service (FSB) said Monday it had detained an American citizen suspected of spying in Moscow and had opened a criminal case against him.

The FSB said the American had been detained on December 28 but it gave no details of the nature of his alleged espionage.

The English-language service of the TASS news agency named the American as Paul Whelan but Reuters was unable to independently confirm the exact spelling of his name.

The Foreign Ministry told TASS it could not provide further detail on the case, but said the U.S. Embassy in Moscow had been informed.