A former US Marine arrested last week on espionage charges in Russia has sought Irish consular assistance.
Canadian-born Paul Whelan is a US citizen and also holds an Irish passport.
He was arrested while he was in Moscow to attend a wedding.
The Department of Foreign Affairs 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."
Mr Whelan is also a British citizen and is seeking assistance from all embassies of countries of which he is a citizen.
A spokesman for the US State Department said yesterday that the US Ambassador in Moscow, Jon Huntsman, had visited Mr Whelan.
"Ambassador Huntsman expressed his support for Mr Whelan and offered the Embassy's assistance," the spokesman said.
UK Foreign Secretary Jeremy Hunt said: "We are extremely worried about Paul Whelan, we have offered consular assistance.
"The US are leading on this because he is a British and American citizen," Mr Hunt added.
Mr Whelan's family said in a statement that they are "deeply concerned for his safety and well-being".
"His innocence is undoubted and we trust that his rights will be respected," they said.
Russia's intelligence service said Mr Whelan was arrested last Friday "while carrying out an act of espionage".
His brother David told CBC News in Canada that there was "no chance" the claim was accurate.
US Secretary of State Mike Pompeo called for an explanation for Mr Whelan's detention and said his release would be demanded if his detention is found to be inappropriate.
"We've made clear to the Russians our expectations that we will learn more about the charges, come to understand what it is he's been accused of and if the detention is not appropriate, we will demand his immediate return," he said in Brasilia where he was attending the presidential inauguration.
Mr Hunt warned Russia not to use UK citizens as pawns in "diplomatic chess games".
He said Paul Whelan was being given "every support that we can" after it emerged that he was a British national.
Speaking during a visit to Singapore, Mr Hunt told Sky News that "individuals should not be used as pawns of diplomatic leverage".
He added: "We are giving him every support that 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 extremely worried about him and his family as we hear this news."
David Whelan said that his brother, who had served in Iraq, has been to Russia many times in the past for both work and personal trips, and had been acting as a tour guide for some of the wedding guests.
His friends filed a missing persons report in Moscow after his disappearance, his brother said.
David Whelan did not immediately respond to requests for additional information.
Paul Whelan is 48 and lives in Novi, Michigan, according to public records.
He served in the US Marine Corps Reserves from 10 May 1994 to 2 December 2008 and the highest rank he attained was staff sergeant, according to records provided by the Pentagon.
He was discharged for bad conduct following his conviction by court martial on charges related to larceny, the Pentagon said.
BorgWarner Inc, a Michigan-based automotive parts supplier, said Paul Whelan is the company's director of global security and oversees the safety of its facility in Auburn Hills, Michigan, and other locations.
According to the company's website, it does not have any locations in Russia.
Mr Whelan's arrest came after Russian President Vladimir Putin accused Western nations of using espionage cases to try to undermine an increasingly powerful Russia.
US intelligence services have accused Russia of interfering in the 2016 presidential election.
Special Counsel Robert Mueller has charged 25 Russians, including members of the GRU military intelligence, and three Russian companies for that alleged interference but they have not been arrested.
In December, Russian national Maria Butina pleaded guilty in a Federal Court in Washington to acting as an illegal foreign agent.
Ms Butina faces up to six months in prison, followed by likely deportation.
Additional reporting AFP/Reuters