Britain cautioned Russian Federation on Friday that individual citizens should be not used as pawns in a diplomatic chess game after Paul Whelan, a dual U.S. -U.K. national who was born in Canada, was detained in Moscow on espionage charges.
British Foreign Secretary Jeremy Hunt said he was "extremely worried" about Whelan and "every support was being offered" to him.
"We urge the U.S. Congress and the State Department to help on Paul's behalf to secure his release and return him home soon".
Hunt said the US was "leading the case" and the United Kingdom has offered consular assistance. Jon Huntsman, the US ambassador to Moscow, visited the American prisoner this week.
Secretary of State Mike Pompeo has said Washington would demand Whelan's release if his detention is not "appropriate".
The FSB has opened a criminal case against Whelan, but has not given any details of his alleged espionage activities.
Whelan, who has denied the charges, could face up to 20 years in prison if found guilty.
Russian security expert Pavel Felgenhauer is perplexed by the incident, and believes the arrest is "suspicious".
US diplomats are now scrambling for information about Whelan's case.
Many in the West question whether the arrest was in retaliation to US conviction of Russian gun rights activist Maria Butina. Butina, 30, is the first Russian national to be convicted of seeking to influence US policy in the run-up to the 2016 election by acting as a foreign agent.
CNN said that 20 years that the accused spy may face in Russian Federation would provide "plenty of time to be swapped for..."
Dmitry Novikov, a first deputy head of the global affairs committee in Russia's lower house of parliament, commenting on a possible swap, said Russian intelligence first needed to finish their investigations.
"If we see the need to make a change, then we'll make it".
Accusations made by former foreign secretary Boris Johnson that the Kremlin was behind the nerve agent poisoning of Sergei Skripal and his daughter Yulia in Salisbury previous year were dismissed by Vladimir Putin.
Since 2017, Whelan has worked for a Michigan-based auto parts company called BorgWarner, most recently as director of global security. BorgWarner, based in Auburn Hill, Michigan, has facilities around the world but none in Russian Federation and he never traveled to the country for business, company spokeswoman Kathy Graham said.
USA media said he had previously worked in security and investigations for the global staffing firm Kelly Services, which is headquartered in MI and has operations in Russian Federation. His brother, David, told United States media outlets that Paul had traveled to Moscow to attend a friend's wedding. "Our focus remains on ensuring that Paul is safe, well-treated, has a good lawyer and is coming home".
Whelan was a U.S. Marine reservist who rose to the rank of staff sergeant during 14 years of service, including two deployments in Iraq. But his military career ended with a court martial in 2008, when he was convicted on charges that included attempted larceny and dereliction of duty. He maintained an active presence on the Russian social media site VKontakte and had forged friendships with Russian service members on the platform. In 2015, he posted the words in Russian: "In Moscow." and accompanied it with a Russian mobile phone number. Family members insist that he is innocent - and was in Russian Federation only to attend a wedding. When he was detained he was staying with the rest of the wedding party at Moscow's upmarket Metropol hotel, the brother said.