The United States wants an explanation of why Russia detained a retired US Marine on spying charges in Moscow and will demand his immediate return if it determines his detention is inappropriate, Secretary of State Mike Pompeo said on Wednesday.
Mr Pompeo, in Brasilia for the inauguration of Brazil's new president, said the US government hoped to gain consular access to Paul Whelan within hours.
"We've made clear to the Russians our expectation 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," Mr Pompeo said.
In Moscow, RIA news agency cited a foreign ministry spokesman as saying Russia has allowed consular access to Mr Whelan. Russia's FSB state security service detained Mr Whelan on Friday and opened a criminal case against him.
The State Department did not immediately confirm that Moscow had provided consular access.
Mr Whelan was visiting Moscow for the wedding of a former fellow Marine and is innocent of the espionage charges against him, his family said on Tuesday.
He had been staying with the wedding party at Moscow's Metropol hotel when he went missing, his brother, David, said.
"His innocence is undoubted and we trust that his rights will be respected," Mr Whelan's family said in a statement released on Twitter on Tuesday.
My brother was detained by the Russian government on Friday as an alleged spy. While the law library + info focus will remain, you may see an increase in off-message topics until we get him safely home. pic.twitter.com/2HIF1UmS1b— David Whelan (@davidpwhelan) January 1, 2019
Russia's FSB state security service said Mr Whelan had been detained on Friday, but it gave no details of his alleged espionage activities. Under Russian law, espionage can carry a prison sentence of between 10 and 20 years.
David Whelan told CNN 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 serving 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.
He declined to comment on his brother's work status at the time of his arrest and whether his brother lived in Novi, Michigan, as address records indicate.
BorgWarner, a Michigan-based automotive parts supplier, said Mr Whelan is the "company's director, global security. He is responsible for overseeing security at our facilities in Auburn Hills, Michigan, and at other company locations around the world."
Daniel Hoffman, a former CIA Moscow station chief, said it was "possible, even likely" that Russian President Vladimir Putin had ordered Whelan's arrest to set up an exchange for Maria Butina, a Russian citizen who pleaded guilty on 13 December to acting as an agent tasked with influencing US conservative groups.
Russia says Butina was forced to make a false confession about being a Russian agent.
Mr Putin told US President Donald Trump in a letter on Sunday that Moscow was ready for dialogue on a "wide-ranging agenda," the Kremlin said following a series of failed attempts to hold a new summit.
At the end of November, Mr Trump cancelled a planned meeting with Putin on the sidelines of a G20 summit in Argentina, citing tensions about Russian forces opening fire on Ukrainian navy boats and then seizing them.
Mr Trump's relations with Mr Putin have been under a microscope as a result of US Special Counsel Robert Mueller's investigation into alleged Russian meddling in the 2016 US presidential election and possible collusion with the Trump campaign.
Moscow has denied intervening in the election. Mr Trump has said there was no collusion and characterized Mr Mueller's probe as a witch hunt.
Russia's relations with the United States plummeted when Moscow annexed the Crimean peninsula from Ukraine in 2014. Washington and Western allies have imposed a broad range of sanctions on Russian officials, companies and banks.