Paul Whelan: Ex-US Marine held on suspicion of spying is UK citizen
Russia must not use UK citizens as "pawns in diplomatic chess", Foreign Secretary Jeremy Hunt has said, after the arrest of a former US Marine with British and American nationality.
Mr Hunt said he was "extremely worried" about Paul Whelan, who has been detained on suspicion of spying.
He added that "every support" is being given to the 48-year-old.
Mr Whelan's family said he was attending a wedding, while Russia said he was "caught spying" in Moscow.
It has been reported that Mr Whelan also holds an Irish passport.
Mr Whelan was born in Canada to British parents and moved to the US as a child. He now lives in Michigan and is director of global security for automotive components supplier BorgWarner.
He has been visiting Russia for business and pleasure for more than a decade, his brother said.
His twin David said he had been told his brother was fine and looking healthy, after US ambassador to Russia Jon Huntsman visited him this week.
In a statement, David said: "Our focus remains on ensuring that Paul is safe, well treated, has a good lawyer, and is coming home.
"We urge the US Congress and the state department to help on Paul's behalf to secure his release and return him home soon."
Mr Hunt said: "Individuals should not be used as pawns of diplomatic leverage. We need to see what these charges are against him and understand whether there is a case or not.
"We are giving every support we can, but we don't agree with individuals being used in diplomatic chess games.
"Because it is desperately worrying, not just for the individual but their families, and we are all extremely worried about him and his family as we hear this news."
He added that the UK had not yet had access to Mr Whelan and that the US was leading on this case.
Asked about reports he is also Irish, the Irish embassy in Moscow said: "The Embassy of Ireland in Moscow has requested consular access to an Irish citizen currently detained in Russia after receiving a request for assistance.
"The Department of Foreign Affairs and Trade will provide all possible and appropriate assistance in relation to this case."
Russia and America have traded spying allegations at regular intervals since the Cold War.
Mr Whelan travelled to Russia on 22 December and planned to fly home on 6 January, but was arrested in Moscow on 28 December.
Under the charge of espionage, he could face up to 20 years in jail if found guilty.
By James Landale, BBC diplomatic correspondent
There is much we still don't know about the detention of Paul Whelan.
What are the precise charges against him? Did he really have a memory stick containing a list of Russian intelligence officials when he was arrested, as claimed by one Russian news agency but not thus far corroborated?
What lies behind his long-standing interest in Russia and engagement with Russian social media? And how many nationalities does he actually have - American and British, yes, but does he also have Irish and Canadian passports?
This lack of certainty about the facts makes Jeremy Hunt's decision to accuse Russia of playing diplomatic chess games all the more interesting.
The foreign secretary's remarks support the premise of speculation in the US that Mr Whelan's arrest is a strategic move by Moscow.
The suggestion - and it is no more than that - is that the former US marine could be exchanged for a Russian woman who was jailed in the United States last month. Maria Butina, a pro-gun activist, pleaded guilty to trying to influence US conservative groups on behalf of the Russian government.
Moscow certainly has a track record of using individuals for unpredictable and aggressive diplomacy.
But Mr Hunt has shown that he, for now at least, is ready to respond robustly at a time when Anglo-Russian relations are still struggling to recover from last year's poisoning incident in Salisbury.
Mr Whelan's lawyer, Vladimir Zherebenkov, said he had appealed to the court against the detention of his client and requested he was released on bail. He added that Mr Whelan remained in good spirits.
A Foreign Office spokesman said: "Our staff have requested consular access to a British man detained in Russia after receiving a request for assistance from him."