Russia is to expel a US diplomat it claims was caught disguised in a blond wig while trying to recruit a Russian agent in Moscow.
Russia’s security services claimed that Ryan Fogle, a third secretary at the US Embassy in Moscow, was a CIA officer.
It said Mr Fogle was carrying special technical equipment, disguises, written instructions and a large sum of money when he was detained overnight.
The Russian Foreign Ministry said Mr Fogle, who has diplomatic immunity, was handed back to US officials, declared persona non grata and ordered to leave Russia immediately.
Russia’s Federal Security Service, the FSB, which is the successor to the Soviet-era KGB, said Mr Fogle was trying to recruit a Russian counter-terrorism officer who specializes in the Caucasus, a region in southern Russia that includes Chechnya and Dagestan.
The suspects in the Boston Marathon bombings are ethnic Chechen brothers and the elder brother spent six months last year in Dagestan, now the centre of an Islamic insurgency.
US investigators have been working with the Russians to try to determine whether Tamerlan Tsarnaev had established any contacts with the militants in Dagestan.
It was the first case of an American diplomat publicly accused of spying in about a decade and seemed certain to aggravate already strained relations between Russia and the US.
Noting recent efforts by the two countries to improve cooperation in countering terrorism, the Russian Foreign Ministry said “such provocative actions in the spirit of the Cold War do nothing to strengthen mutual trust”.