Russia has accused the United States of "crude and clumsy" spying after a suspected CIA agent was caught in Moscow seeking to recruit an agent while disguised in a blonde wig.
The foreign ministry issued an official protest to the US ambassador.
The FSB security service said the suspect was caught on Monday as he tried to recruit a Russian security agent with an advance of $US100,000 for intelligence on the Northern Caucasus.
The man was carrying a "typical espionage arsenal" of money and disguises like wigs and spectacles. He is said to have been working undercover as a low-ranking third secretary at the US embassy.
The agent, named as Ryan C Fogle, was handed back to the US embassy and ordered to leave the country after being declared persona non grata.
Kommersant, a daily newspaper, said Fogle was likely seeking intelligence about the Boston marathon bombers whose origins were in the Russian Northern Caucasus
