An alleged CIA agent has been briefly detained in Moscow for allegedly trying to recruit a Russian intelligence officer, Russian media report.
The man, named as US diplomat Ryan Fogle, was held overnight before being released to US officials, Russia's Federal Security Service (FSB) said.
Russia has declared him "persona non grata" and ordered his expulsion, the foreign ministry said on its website.
The US ambassador to Moscow, Michael McFaul, had been summoned, it added.
Mr Fogle is said to have worked as third political secretary at the US embassy in Moscow, which has issued no comment on the matter so far.
Photos have emerged of his alleged detention.
The agent was reportedly arrested with a large sum of money, technical devices and written instructions for the agent he had tried to recruit.
While it is a sensitive time for US-Russian relations because of the crisis in Syria, the FSB's claims to have unearthed a CIA agent are unlikely to have any long-term political consequences, the BBC's Steve Rosenberg reports from Moscow.
Both countries recently said they wished to step up security co-operation after the Boston Marathon bombing.
FSB photos purporting to show Mr Fogle during and after his detention appeared on Russian online media, such as these published by broadcaster RT.
Wearing a blue checked shirt and a plain baseball cap, he is shown being held on the ground with his hands bound, then being escorted away.
Another photo shows him sitting at desk, his hat removed.
Possessions said to be Mr Fogle's are laid out on a table. They include a sum of money in 500-euro banknotes and two wigs, one of which he was apparently wearing at the time of his detention.
Also on the table are a compass, map, knife, dark glasses and small mobile phone.
"FSB counter-intelligence agents detained a CIA staff member who had been working under the cover of third political secretary of the US embassy in Moscow," the FSB said.
"At the moment of detention, special technical equipment was discovered, written instructions for the Russian citizen being recruited, as well as a large sum of money and means for altering appearance."
Russian state TV has displayed a piece of paper, which it said was Mr Fogle's letter to the Russian officer.
Addressing the recipient as "Dear friend", the letter offers $100,000 (£65,400) "to discuss your experience, expertise and co-operation".
It goes on to say: "We can offer up to $1m a year for long-term co-operation, with extra bonuses if we receive some helpful information.
"This is a down-payment from someone who is very impressed with your professionalism and who would greatly appreciate your co-operation in the future."
The letter is simply signed "Your friends".