“Such events certainly should not affect the top-level agreements on serious issues: the situation in Syria, the Korean Peninsula, Afghanistan and rearmament,” Klimov said.

Russia and the United States have a “strategic relationship” he said, adding the two nations relations had weathered a number of more “serious” cases before. “The work of special services has never triggered positive emotions,” he added.

On Tuesday, the Federal Security Service (FSB) said Ryan Christopher Fogle, a US diplomat working as a third secretary in the embassy’s political department, was detained on the night of May 13 as he attempted to recruit an officer from one of Russia's special services. The officer concerned was involved in counterterrorism in the volatile North Caucasus, media reports said.

Fogle allegedly offered the Russian officer up to $1 million a year for “long-term cooperation” and additional bonuses for valuable information. His "spy kit" shown on television included two wigs, a letter setting out a recruitment deal, a compass, a Moscow atlas and smartphones.

Later on Tuesday the diplomat was declared persona non grata by Russia's Foreign Ministry. The State Department has confirmed that an officer was detained and later released by Russian officials.

US Ambassador Michael McFaul visited the Foreign Ministry in downtown Moscow on Wednesday morning, having been summoned the day before over the issue. The Foreign Ministry later issued a statement saying Russia had lodged an official protest with Washington over the matter.

In an interview with the Golos Rossii radio, former FSB Director Nikolai Kovalyov described the Fogle incident as a “failure for the Americans” and “a success for [Russia’s] counterintelligence.”