Russia on Wednesday accused the United States of "crude and clumsy" spying on its territory 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 who it summoned to an early morning meeting, but signs also grew that neither side wanted the Cold War-style incident to develop into a full scale crisis.
The suspected agent was caught red-handed late Monday as he tried to recruit a Russian security agent with an advance of $100,000 for intelligence on the Northern Caucasus, according to the Russian FSB security service.
The man, who was carrying a "typical espionage arsenal" of money and disguises like wigs and spectacles, is said to have been working undercover as a low-ranking third secretary at the US embassy.
"To say the least, we are surprised by the extremely crude and clumsy recruitment" that came after pledges by both sides to improve cooperation, said President Vladimir Putin's foreign policy advisor Yuri Ushakov, quoted by ITAR-TASS.
Ushakov also expressed surprise that it appeared vows by Putin and President Barack Obama for special services to work closer together had not filtered through to the CIA or the US embassy.
But Ushakov also announced that Russian national security chief Nikolai Patrushev would visit the United States May 20-21, possibly carrying a message on bilateral relations from Putin to Obama.
He said: "I do not think that what has happened will influence this cooperation, all the more as its importance has been agreed at a high level."
The suspected 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.
The Kommersant daily said that Fogle was likely to have been seeking intelligence about the Boston marathon bombers whose origins were in the Russian Northern Caucasus, despite pledges by security services to cooperate in the investigation.
It said Fogle may have been trying to recruit a Russian anti-terror agent in Dagestan whose details US officials had obtained on a visit to the Caucasus in April that was facilitated by the Russian side.
The foreign ministry said that Deputy Foreign Minister Sergei Ryabkov had issued a formal protest to US ambassador to Moscow Michael McFaul.
In Washington, State Department acting deputy spokesman Patrick Ventrell confirmed McFaul had met with Ryabkov, but refused to go into the details of a "private diplomatic conversation".
Ventrell would also not divulge whether the US embassy official had now left Russia, saying only "we do comply with our Vienna Convention obligations and requirements when countries ask us to send someone home".
It also remained unclear whether Washington intended to retaliate with a tit-for-tat move against a Russian diplomat based in the United States.
An FSB agent interviewed by Russian state television meanwhile said Moscow had already warned the United States against spying activities, but "they did not listen to us".
"Fogle arrived in Spring 2011. Russia already had intelligence he was from the CIA and from the moment of his arrival he was put under the corresponding operative control," said the agent, whose voice was distorted and face kept in shadow to keep his identity secret.
Nevertheless, both sides appeared keen to avoid inflammatory rhetoric at a time when they are engaged in sensitive diplomacy aimed at ending the conflict in Syria.
Russian Foreign Minister Sergei Lavrov steered clear of the topic, saying that he had opted not to bring up the case at talks in Sweden with US Secretary of State John Kerry.
Former head of the FSB Nikolai Kovalyov described the alleged agent's interception as a great success for Russian intelligence as it was rare for spies to be caught red-handed, "all the more with such attributes as a wig".
Kovalyov, now a ruling party lawmaker, however predicted the episode would have no effect on bilateral relations.
"The Americans do nothing other secret services -- including ours -- would not do," he told the Interfax news agency.