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 Russia's Federal Security Service on Dec. 28. His family have said he is innocent and that he 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 US are leading on this because he is a British and American citizen."
Official sources familiar with US and British intelligence told Reuters on Friday that they were unaware of any possible connections between Whelan and any US or British agency.