(CNN) Russia has detained a US citizen on suspicion of spying, according to the Russian Federal Security Service (FSB).

American Paul Whelan was arrested December 28 in Moscow "while carrying out an act of espionage," the FSB said in a statement.

"The investigation department of the Federal Security Service of Russia initiated a criminal case against a US citizen under article 276 of the Criminal Code of the Russian Federation. The investigation is underway," the statement continued. Article 276 is espionage.

The Russia Ministry of Foreign Affairs "formally notified" the United States of the detention, a US State Department spokesman said.

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