Russia must not use UK citizens as “pawns in diplomatic chess” the foreign secretary has said, after the arrest of joint US-UK national Paul Whelan.
Jeremy Hunt said he was “extremely worried” about the former US Marine, 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.
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.
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.
BBC diplomatic correspondent James Landale said one Russian news agency suggested Mr Whelan was found in possession of a memory stick, allegedly containing the names of intelligence officials.
He also said there had been speculation Mr Whelan was detained so he could be exchanged with Maria Butina, a Russian gun rights activist who was jailed in the US last month.
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, 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 as well?
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 United States 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 Maria Butina, a Russian woman who was jailed in the United States last month.
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 has appealed to the court against the detention of his client and requested he is 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.”
RTE reported that Mr Whelan also holds an Irish passport and had sought Irish consular assistance.