MOSCOW (AP) - A US diplomat was ordered to leave the country after the Kremlin's security services said he tried to recruit a Russian agent, and they displayed tradecraft tools that seemed straight from a cheap spy thriller: wigs, packets of cash, a knife, map and compass, and a letter promising millions for "long-term cooperation."

The FSB, the successor agency to the Soviet-era KGB, identified the diplomat as Mr Ryan Fogle, a third secretary at the US Embassy in Moscow, detaining him briefly overnight.

It alleged that Mr Fogle was a CIA officer trying to recruit a Russian counter-terrorism officer who specialises in the volatile Caucasus region in southern Russia, where the two Boston Marathon bombing suspects had their ethnic roots.

Mr Fogle was handed over to US Embassy officials on Tuesday, declared persona non grata and ordered to leave Russia immediately. He has diplomatic immunity, which protects him from arrest.

The State Department would only confirm that Mr Fogle worked as an embassy employee, but wouldn't give any details about his employment record or responsibilities in Russia. Some officials also referred inquiries to the CIA, which declined comment.

Mr Fogle was the first American diplomat to be publicly accused of spying in Russia in about a decade. While relations between the two countries have been strained, officials in both Washington and Moscow sought to play down the incident.

The Russian Foreign Ministry summoned US Ambassador Michael McFaul to appear on Wednesday in connection with the case. Mr McFaul said he would not comment on the spying allegation.

Russian officials expressed indignation the US would carry out an espionage operation at a time when the two countries have been working to improve counterterrorism cooperation.

"Such provocative actions in the spirit of the Cold War do nothing to strengthen mutual trust," the Foreign Ministry said.

Russia's Caucasus region includes the provinces of Chechnya and Dagestan. The suspects in the April 15 Boston Marathon bombings - Dzhokhar Tsarnaev and his elder brother, Tamerlan, who was killed in a manhunt - are ethnic Chechens. Tamerlan 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 militants in Dagestan.

Despite the end of the Cold War, Russia and the United States still maintain active espionage operations against each other. Last year, several Russians were convicted in separate cases of spying for the US and sentenced to lengthy prison sentences.

But Tuesday's case had espionage elements that seemed more like "Spy vs Spy" than Ludlum and le Carre.

Russian state TV showed pictures of a man said to be Mr Fogle, wearing a baseball cap and a blond wig, lying face down on the ground. The man, without the wig, was also shown sitting at a desk in the offices of the FSB, the Federal Security Service.

Two wigs, a compass, a map of Moscow, a pocket knife, three pairs of sunglasses and envelopes of 500 euro notes were among the items the FSB displayed on a table.

The FSB also produced a typewritten letter that it described as instructions to the Russian agent who was the target of Mr Fogle's alleged recruitment effort.

The letter, in Russian and addressed "Dear friend," offers US$100,000 (S$124,037) to "discuss your experience, expertise and cooperation" and up to US$1 million a year for long-term cooperation.

The letter also includes instructions for opening a Gmail account to be used for communication and an address to write. It is signed "Your friends."

"If this is genuine, then it'll be seen to be appallingly bad tradecraft - being caught with a 'How-to-be-a-Spy 101' guide and a wig. He would have had to have been pretty stupid," said Mark Galeotti, a professor at New York University who studies the Russian security services.

Mr Samuel Greene, director of the Russia Institute at King's College London, called the evidence bizarre.

"I wouldn't have thought that spies gave each other written instructions," he said in a telephone interview. Mr Greene also noted that the FSB had displayed Mr Fogle's official diplomatic ID, suggesting he was carrying it along with the spy paraphernalia when he was detained.

"Maybe this is what the CIA has come to, maybe the propaganda folks in the Kremlin think we are this stupid, or maybe both," he said.

A five-minute video produced by the FSB and shown on state TV showed a Russian official speaking to what appear to be three US diplomats who had come to pick up Mr Fogle in the FSB office.

The official, whose face is blurred, alleged that Mr Fogle called an unidentified FSB counter-intelligence officer who specialises in the Caucasus at 11:30 pm on Monday. He then said that after the officer refused to meet, Mr Fogle called him a second time and offered 100,000 euros if he would provide information to the US.

The Russian official said the FSB was flabbergasted. He pointed to high-level efforts to improve counterterrorism cooperation, specifically FBI director Robert Mueller's visit to Moscow last week and phone calls between President Barack Obama and Russian President Vladimir Putin.

"At a time when the presidents of the two countries are striving to improve the climate of relations between the two countries, this citizen, in the name of the US government, commits a most serious crime here in Moscow," the official said.

State Department spokeswoman Jen Psaki confirmed that an officer at the US Embassy in Moscow was briefly detained and released.

"We have seen the Russian Foreign Ministry announcement and have no further comment at this time," said Psaki, who was in Sweden with Secretary of State John Kerry.

Little was immediately known about Mr Fogle. A third secretary is an entry level position at the State Department, the lowest diplomatic rank in the foreign service.

Mr Putin has stoked anti-American sentiments among Russians in recent years in what is seen as an effort to build support at home. He also appears to have a genuine distrust of Russian non-governmental organisations that receive American funding, which he has accused of being fronts that allow the US government to meddle in Russia's political affairs. Hundreds of NGOs have been searched this year as part of an ongoing crackdown by the Russian government.

Mr Galeotti said the public exposure of Mr Fogle suggests a political purpose behind the detention. He said these kinds of spying incidents happen with some frequency, but making such a big deal of them is rare.

"More often, the etiquette is that these things get dealt with quite quietly - unless they want to get a message out," Mr Galeotti said.

"If you identify an embassy staffer who is a spy for the other side, your natural impulse is to leave them be, because once you identify, you can keep tabs on them, see who they talk to and everything else." "There's no reason to make a song and dance, detain them, eject them," he said.

Mr Greene said Mr Fogle's detention should be seen as part of Mr Putin's confrontation with the opposition and not as something likely to have a major impact on US-Russia relations.

"I think this is mostly for domestic consumption in Russia so that people say, 'look at these naughty Americans trying to meddle in our internal affairs and spy on us,"' Mr Greene said. "But everybody's got spies everywhere so I don't see this as a major issue."

In Washington, State Department spokesman Patrick Ventrell also said the incident was unlikely to hamper US-Russia relations.

"I'm not sure I'd read too much into one incident one way or another," he told reporters, and pointed to Mr Kerry's meeting with Russian Foreign Minister Sergey Lavrov in Sweden on Tuesday evening. "We have a very broad and deep relationship with the Russians across a whole host of issues, and we'll continue to work on our diplomacy with them directly."

Mr Alexei Pushkov, who heads the international affairs committee in Russia's parliament, wrote in a Twitter post that the spy scandal would be short-lived and would not interfere in Mr Kerry and Mr Lavrov's discussions aimed at bridging deep differences over the civil war in Syria.

"But the atmosphere is not improving," Mr Pushkov commented.