A photo taken on March 2, 2018 shows the headquarters of the FSB security service in Moscow. (Photo: AFP)

MOSCOW, Dec. 31 (Xinhua) -- The Russian Federal Security Service (FSB) said Monday that it has detained US citizen Paul Whelan here 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.

Russian President Vladimir Putin said that Butina has no ties to Russian intelligence services, and that the whole case against her was unfounded.