An American former Marine who is being held in Moscow on spying charges also holds British, Irish and Canadian citizenship, officials say.
Britain's foreign secretary accused Russia on Friday of trying to use Paul Whelan as a pawn in its geopolitical games.
Both Ireland and the UK have asked that their diplomats be allowed to visit him.
The news that Whelan holds citizenship in four countries adds complexity to an already-murky case, and brings international pressure on Russia from several fronts.
Whelan, the 48-year-old global security director for a US auto parts company, was arrested a week ago in Moscow. At the time, he was identified only as an American.
Russian authorities have released no information about the charges against Whelan, who could face up to 20 years in prison if convicted of spying.
Russian media reported Thursday that Whelan had been formally indicted for spying and the Interfax news agency said he denied the allegation.
Whelan's family says he was in Russia to attend a friend's wedding. A Russian lawmaker, meanwhile, hinted Friday that the detainee could possibly be swapped for a Russian woman who has pleaded guilty to trying to influence US politics.
Relations between Moscow and London have hit a low point in the wake of Britain's allegations that Russian military intelligence agents were behind the nerve-agent poisoning of a Russian former double agent and his daughter in the British city of Salisbury in March.
British Foreign Secretary Jeremy Hunt said his government was helping Whelan.
"We are giving him every support we that we can, but we don't agree with individuals being used in diplomatic chess games," Hunt said Friday on Sky News.
"We are extremely worried about him and his family."
Whelan's British citizenship was reported by the US embassy to British officials on Thursday, according to Britain's Press Association. That was a day after US Ambassador Jon Huntsman Jr. met with Whelan at Lefortovo Prison in Moscow.
"He has British citizenship. The British side has sent a request for a consular visit. Work on it is in progress," the Russian state news agency Tass cited Foreign Ministry spokeswoman Maria Zakharova as saying.
Ireland's Department of Foreign Affairs on Friday confirmed that Whelan also holds Irish citizenship and said it also is requesting consular access to him in Moscow.
Asked about Whelan, Canadian Global Affairs spokesman Richard Walker said "consular officials are aware that a Canadian citizen had been arrested in Russia."
Whelan's twin brother David said in a statement for the family that they were "very pleased to know that staff of the U.S. Embassy in Moscow have been given consular access to Paul and confirmed that he is safe."
He said the family's "focus remains on ensuring that Paul is safe, well treated, has a good lawyer, and is coming home." He urged both the U.S. Congress and the State Department to help get his brother freed.
