Russian Foreign Ministry spokeswoman Maria Zakharova said on Friday that Paul Whelan, who was detained on December 28 in Moscow "while carrying out an act of espionage," holds a UK passport in addition to his American travel document.
She added that British diplomats have sent a request for consular visits, Presstv Reported.
Russia's FSB state security service opened a criminal case against the 48-year-old, whose family denied the charges, saying he was visiting Moscow for the wedding of a retired Marine to a Russian woman.
On Friday, Britain slammed Russia for detaining Whelan, accusing Moscow of using British citizens as diplomatic pawns.
"We don't agree with individuals being used in diplomatic chess games... We are all extremely worried about him and his family," Foreign Secretary Jeremy Hunt told the BBC in an interview, adding, "We need to see what these charges are against him."
Diplomatic ties between Russia and the UK are tense, particularly following a nerve agent attack in March 2018 on a former spy in the city of Salisbury.
Britain alleged that Russia had ordered the poisoning of Sergei Skripal, a former double spy, and his daughter in southern England.
Russia has denied any involvement and even some officials in Moscow believe the entire story, which led to a serious political fallout with the Western countries, was fabricated by the British intelligence agencies to target Russia.
Whelan was reportedly visited by US Ambassador to Russia Jon Huntsman on Wednesday.
US Secretary of State Mike Pompeo said Washington wants a clarification regarding Whelan's arrest and will demand his immediate return if it determines his detention is inappropriate.
If convicted, Whelan could face up to 20 years in prison.
Since late 2016, US spying agencies have been accusing Russia of meddling in the American presidential election.
Back then, Washington alleged that Russia had influenced the election in favor of Donald Trump, running a massive secret social media trolling and targeted messaging operation, an allegation Moscow strongly denied. The US has imposed a series of unilateral sanctions against Moscow over the claims.