Russia summoned US ambassador to Russia Michael McFaul to the Foreign Ministry in Moscow on Wednesday after expelling an American embassy employee accused of trying to recruit a Russian intelligence officer to work for the CIA.
Moscow said he had been caught red-handed with disguises, special equipment and wads of cash.
McFaul made no comment as he left the meeting.
The incident came at an awkward time for the two countries, as they try to improve relations and bring the warring sides in Syria together for an international peace conference.
Moscow did not threaten to take any further action.