The reasons for the reported detention of Siamak Namazi were not immediately known, though hard-liners in Iran's judiciary, military and intelligence services have warned against easing hostilities with the US.
Today, Iran and the US sat down together in new multilateral talks with other countries, trying to end Syria's long civil war.
In addition to Namazi's detention, an Internet freedom group said a Washington-based Lebanese citizen recently disappeared while on a trip to Tehran.
Attempts by The Associated Press to reach family members of Namazi, who has a home in Washington, weren't immediately successful today.
Iranian officials could not be reached for comment today, part of the Iranian weekend, and state media did not report on Namazi's reported detention.
The US State Department declined to confirm Namazi's arrest.
"We're aware of recent reports of the possible arrest in Iran of a US citizen. We're looking into these reports and don't have anything further to provide at this time," Michael Tran, a State Department spokesman, said late yesterday.
He later fled as a child with his family after the 1979 Islamic Revolution. But the family appears to have kept business ties in Iran, and Namazi travelled back several times.
His last known employer was Crescent Petroleum, a firm based in Sharjah in the United Arab Emirates. A representative of the company said Namazi traveled to Tehran in mid-July on a private visit and "at the time of his reported detention was no longer an employee of the company."
A consulting company in Iran with which Namazi previously had ties, Atieh Bahar, denied to AP that he had any ongoing role there.
