WASHINGTON, Dec. 31 (Xinhua) -- The U.S. State Department said on Monday that it has been notified of the detention of an American citizen by Russia, whom Moscow had accused of spying.

"We are aware of the detention of a U.S. citizen by Russian authorities," a State Department spokesperson said in a statement, adding that it has been "formally notified of the detention" by Russian Foreign Ministry.

The spokesperson also noted that the United States has requested consular access and expected "Russian authorities to provide it."

The Russian Federal Security Service (FSB) said Monday that it detained U.S. citizen Paul Whelan for spying three days ago.

The suspect was caught when he was "conducting a spy action" on Friday in the Russian capital, the FSB said in a statement, without giving more details.

The agency has initiated a criminal case against Whelan.

The detention came after Russian citizen Maria Butina pleaded guilty in a Washington court on Dec. 13 to one conspiracy charge, admitting that she acted as an unregistered foreign agent to advance Moscow's interests.