London/Moscow - Britain cautioned Russia on Friday that individuals should not be used as diplomatic pawns, after a former US marine who also holds a British passport was detained in Moscow on espionage charges.
Paul Whelan was arrested by the FSB state security service last Friday. His family have said he is innocent and was in Moscow to attend a wedding.
"Individuals should not be used as pawns of diplomatic leverage," British foreign minister Jeremy Hunt said.
"We are extremely worried about Paul Whelan, we have offered consular assistance," Hunt said. "The U.S. are leading on this because he is a British and American citizen."
US Secretary of State Mike Pompeo said this week that the United States had asked Russia to explain Whelan's arrest and would demand his immediate return if it determined his detention is inappropriate.
The FSB has opened a criminal case against Whelan but not given any details of his alleged activities. In Russia, an espionage conviction carries a sentence of between 10 and 20 years in prison.
Whelan's family have said that he was visiting Moscow for the wedding of a retired Marine.
The detention further complicates a strained relationship between Moscow and Washington, despite the professed desire of the two presidents, Donald Trump and Vladimir Putin, to build a personal rapport.
US intelligence officials accuse Russia of meddling in U.S. elections -- a charge Russia denies.
Russian national Maria Butina admitted last month to U.S. prosecutors that she had tried to infiltrate American conservative groups as an agent for Moscow.
David Hoffman, a former CIA Moscow station chief, said it was "possible, even likely" that Russia had detained Whelan to set up an exchange for Butina.
TOXIC RELATIONS
Whelan's British citizenship introduces a new political dimension: ties between London and Moscow have been toxic since the poisoning in Britain of former Russian spy Sergei Skripal and his daughter.
Britain alleges Skripal was poisoned by Russian intelligence agents posing as tourists, while Russia denies any involvement.
Whelan is director of global security at BorgWarner, a U.S. auto parts maker based in Michigan.
The company said in a statement that Whelan was "responsible for overseeing security at our facilities in Auburn Hills, Michigan, and at other company locations around the world". Its website lists no facilities in Russia.
U.S. media reported that he had previously worked in security and investigations for the global staffing firm Kelly Services, which is headquartered in Michigan and has operations in Russia.
Whelan's military record, provided by the Pentagon, showed that he had served in the U.S. Marine Corps for 14 years until he was discharged in 2008 after being convicted on charges related to larceny.