"If the detention is not appropriate we will demand his immediate return," Pompeo told reporters while on a visit to Brazil, where he was attending the inauguration of that country's new government.

Russia's FSB security service said Monday it had detained the American, who he said was "carrying out an act of espionage."

The man was identified as Paul Whelan, 48, a former Marine now employed as director of global security at US-based automotive components supplier BorgWarner.

Whelan's family said he was visiting Moscow to attend a wedding and denied he was a spy.

Pompeo said in Brasilia: "We are hopeful within the next hours we will get consular access to see him and get a chance to learn more."

He added that "we have made clear to the Russians our expectation that we will learn more about the charges and come to understand what it is he has been accused of."