The US ambassador to Russia was summoned by the Foreign Ministry over the case of an alleged CIA agent uncovered by Moscow's secret services.
Russia's Federal Security Service (FSB) arrested US diplomat Ryan Fogle disguised in a blond wig, as he was allegedly trying to recruit a counterintelligence officer for Washington.
Ambassador Michael McFaul entered the Ministry building in central Moscow in the morning and left after about half hour without saying anything to journalists that were waiting outside.
The Russian foreign ministry said Fogle has been labelled as persona non grata and is to be expelled from the country. Officially an US embassy employee, Fogle is protected by diplomatic immunity.
According to FSB, Fogle was caught carrying "special technical equipment, written instructions for recruiting a Russian citizen, a large sum of money and means for changing a person's appearance," when arrested.
However the content of his 'spy kit' has raised doubts among many security experts.
To complete his secret mission, described by the Moscow as a provocative act "in the spirit of the Cold War", Fogle was armed with two wigs, a Moscow map, sunglasses, a knife, an envelope full of €500 notes and a letter singed "Your Friends" detailing instructions to set up a Gmail account and offering up to $1m (£650,000) for his cooperation.
"I very much doubt that a highly trained CIA operative is going to be walking the streets of Moscow wearing a really bad blonde wig," said CNN counter-intelligence expert Eric O'Neill. "It looks like a set-up to me."
"I'm just surprised that the guy was such an idiot," Russian investigative journalist Yevgenia Albats, who penned a book on the KGB, told the New York Times. "Why did he have to do it in such an old-fashioned way? It sounds like the '70s."
Fogle's arrest was announced at the same time McFaul, was about to start a bilingual Q&A session on Twitter to reach out to a Russian audience on thorny issues that have been straining diplomatic relations between the Kremlin and Washington in recent months.
The #AskMcFaul initiative was hijacked by questions on the erupting scandal which the diplomat refused to comment on.
McFaul has had a tough time in Moscow since he took office in January 2012. His first act as Ambassador was to invite a group of opposition activists and human rights advocates to the embassy, enraging Moscow.
The US State Department confirmed that Fogle worked as an embassy employee, but did not disclose any other detail. The CIA also refused to comment.