The US is hoping to get consular access “within the next few hours” to see US citizen Paul Whelan who was arrested in Russia on Friday for spying.
Secretary of State Mike Pompeo told reporters that they wanted to know more about the charges and “if the detention is not appropriate, we will demand his immediate return”.
Russia says Mr Whelan was “caught spying” in Moscow.
His family says he was in Russia to attend a wedding and is innocent.
Spy scandals have erupted between Russia and America at regular intervals since the Cold War, while Russia’s actions in Ukraine since 2014, and allegations of Russian meddling in the 2016 US presidential election, have seen relations plummet.
Who is Paul Whelan?
Mr Whelan is a 48-year-old former Marine who is now director of global security for Michigan-based automotive components supplier BorgWarner.
His twin brother David Whelan told BBC News that Paul had arrived in Russia on 22 December and had been due to return on 6 January.
He had been attending the wedding of a fellow former Marine to a Russian citizen and had planned to visit Russia’s second city, St Petersburg, in addition to Moscow.
Paul Whelan, his brother said, has been visiting Russia for business and pleasure since 2007.
David said Paul would stand out in a crowd as he is “about six foot [1.8 metres] and kind of hefty with a former soldier’s build”. But asked if he could think of any reason why he had attracted the attention of Russian security services, David Whelan was adamant there was none.
“I can’t imagine how someone with a law enforcement background who is also a former US Marine, and who is now working in corporate security and is also aware of the risks of travel, would have broken any law let alone the law related to espionage,” he said.
“His innocence is undoubted and we trust that his rights will be respected,” the family said in its statement.
What is Paul Whelan alleged to have done?
Russia’s FSB state security agency has given few details, saying only that Mr Whelan was detained “during an act of espionage”, a wording which implies that he was caught red-handed, the BBC’s Sarah Rainsford reports.
He was arrested in Moscow on 28 December and has been charged with espionage, which if found guilty could result in a maximum jail term of 20 years.
How extensive is spying between Russia and the US?
The two countries have been spying on each other for decades but very few US citizens have been arrested for espionage on Russian territory:
- In 2013, US diplomat Ryan Fogle was arrested and expelled after being accused of trying to recruit a Russian intelligence officer as a spy
- In 2000, former naval intelligence officer Edmond Pope was tried and convicted of espionage but pardoned by Russian President Vladimir Putin
The two countries have expelled each other’s diplomats at intervals, notably last year over the nerve agent attack in the UK, which was blamed on Russia.
Last month, a Russian gun rights activist held in the US, Maria Butina, pleaded guilty to conspiracy. US prosecutors say she acted as a Russian state agent, infiltrating conservative political groups.
In 2010, 10 Russian agents were arrested in the US for deep-cover espionage and later swapped for four Russians convicted of spying for the West.