Moscow city skyline (Picture: PA).
The Russian government said it has allowed an American citizen held on espionage charges to have access to US officials seeking answers about his arrest.
US consular officials were granted access to Paul Whelan for the first time since his arrest during a visit to the country, according to a Russian Foreign Ministry spokeswoman quoted by the state news agency Tass.
Paul Whelan, who is head of global security for a Michigan-based car parts supplier, was arrested on Friday.
In announcing the arrest three days later, the Russian Federal Security Service said he was caught "during an espionage operation" but gave no details.
Secretary of state Mike Pompeo said earlier on Wednesday that the US hoped to gain access soon to the former marine and that "if the detention is not appropriate, we will demand his immediate return".
Mr Whelan, 48, was in Moscow to attend a wedding when he disappeared, his brother David Whelan said.
Mr Pompeo said the Trump administration has "made clear to the Russians our expectation that we will learn more about the charges and come to understand what it is he's been accused of".
Mr Whelan's family said in a statement which David Whelan posted on Twitter: "We are deeply concerned for his safety and well-being.
"His innocence is undoubted and we trust that his rights will be respected."
The Russian spying charges carry a prison sentence of up to 20 years.
David Whelan said in an interview that his brother had been to Russia several times previously, so when a fellow former Marine was planning a wedding in Moscow with a Russian woman he was asked to come along to help out.
The Marine Corps on Wednesday released details of Paul Whelan's service record.
He served in the Marine Corps Reserves from 1994 to 2008 and was convicted of larceny in a special court martial in January 2008.
The Marine Corps did not provide details of the court martial conviction beyond saying it was based on "several charges related to larceny".
Mr Whelan attained the rank of staff sergeant in December 2004 after the first of two deployments in Iraq.
He was an administrative clerk and administrative chief.
Mr Whelan was given a bad-conduct discharge in December 2008 at the rank of private.
His last place of duty was at Marine Corps Air Station Miramar in California.
David Whelan said in an interview that his brother had been to Russia several times previously, so when a fellow former marine was planning a wedding in Moscow he was asked to go along to help out.
The morning of his arrest, he had taken a group of wedding guests on a tour of the Kremlin museums.
The last time anyone heard from him was at about 5pm and then he failed to show up that evening for the wedding, his brother said.
"It was extraordinarily out of character," he said.
The family feared he had been mugged or was in a car accident, David Whelan said, and it was when searching the internet on Monday that he learned of the arrest.
"I was looking for any stories about dead Americans in Moscow, so in a way it was better than finding out that he had died," he said.
David Whelan said he has no idea why his brother was targeted by the Russian security services. Paul Whelan had travelled to Russia in the past for work and to visit friends he had met on social networks, his brother said.
Paul Whelan did multiple tours in Iraq with the Marine Corps, his brother said. He now lives in Novi, Michigan, and is director of global security for BorgWarner, where he has worked since early 2017.
"He is responsible for overseeing security at our facilities in Auburn Hills, Michigan and at other company locations around the world," company spokeswoman Kathy Graham said in a statement.
She said BorgWarner does not have any facilities in Russia.