The US ambassador in Russia, Jon Huntsman, has visited Paul Whelan, the ex-marine detained on suspicion of spying.
The US state department confirmed Mr Huntsman went to Lefortovo detention centre in Moscow.
Mr Whelan's brother David said consular staff told him his brother was fine and looked healthy.
Earlier, US Secretary of State Mike Pompeo said the US wanted to know more about the charges facing Mr Whelan.
"If the detention is not appropriate, we will demand his immediate return."
Russia says Mr Whelan was "caught spying" in Moscow but his family says he was in Russia to attend a wedding and is innocent.
Russia and America have traded spying allegations 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 was born in Canada but moved to the US as a child. He is currently director of global security for Michigan-based automotive components supplier BorgWarner.
His twin brother David Whelan said he had been visiting Russia for business and pleasure since 2007.
According to his service record, released on Wednesday by the US Marine Corps, Paul Whelan joined the Marine Reserves in 1994 and rose to the rank of staff sergeant in 2004. He served in Iraq for several months in 2004 and 2006.
He was convicted in a 2008 court martial on charges related to larceny and received a bad-conduct discharge. Details of the charges were not given.
The Whelan family have not commented on the court martial revelations, but had earlier said "his innocence is undoubted" on the Russian allegations.
"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," David Whelan said.
What were Paul Whelan's movements in Russia?
Paul Whelan arrived in Russia on 22 December to attend 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 before flying home on 6 January, his brother said.
He was arrested in Moscow on 28 December, having taken a group of wedding guests on a tour of the Kremlin museums in the morning. He was last heard from in the early evening and failed to show up for the wedding, David Whelan said.
Paul Whelan has been charged with espionage and, if found guilty, he could face up to 20 years in jail.
Russia's FSB state security agency has given few details, saying only that he was detained "during an act of espionage", a wording which implies that he was caught red-handed, the BBC's Sarah Rainsford reports.
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
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.