The detention of Kian Tajbakhsh comes amid a crackdown on NGOs at a time when Iranian authorities accuse the United States of using critics and dissidents to overthrow the country's hard-line government.
Tajbakhsh, an urban planning consultant who has also worked for the World Bank, was detained on or about May 11, the Open Society Institute — a private foundation that encourages democracy-building — said in a statement.
"We are concerned for his safety and call for his immediate release," the statement said, adding that the 45-year-old Tajbakhsh has worked as a consultant for the institute since 2004 "with the knowledge of the Iranian government."
Iranian officials could not be reached to comment on the detention.
Earlier this month, Iranian authorities earlier this month arrested Haleh Esfandiari, director of the Middle East Program at the Washington-based Woodrow Wilson Center for Scholars. Esfandiari, also an Iranian-American, was charged Monday with setting up a network to overthrow the Islamic establishment, the government announced.
The Wilson Center denied the charges. "Haleh is a scholar and has never been a spy," said Lee H. Hamilton, the center's president and director. Esfandiari's work had involved bringing Iranian scholars and others to the U.S. to give talks on the situation in Iran.
The detentions come as tensions have mounted between the United States and Iran, even as the two countries prepare to hold ambassador-level talks in Baghdad on calming Iraq's violence.
The United States accuses Iran of backing militants in Iraq and of seeking to build nuclear weapons, accusations that Tehran denies. U.S. Navy ships with 17,000 sailors and Marines moved into the Persian Gulf on Wednesday for air training exercises in a show of military force off Iran's coast.
Esfandiari had been in Iran visiting her ailing 93-year-old mother but was prevented from leaving in December, when her passport was stolen. She was interrogated repeatedly before her eventual arrest and has since been held at Iran's notorious Evin prison, where dissidents are often held.
When it announced the charges against Esfandiari, the Iranian Intelligence Ministry accused Soros' New York-based Open Society Institute of being involved in the plot to set up the network to overthrow the government.
The institute said Wednesday that Tajbakhsh had been consulting "to facilitate public health, humanitarian assistance and urban planning projects that we undertook openly and with the knowledge of the Iranian government." He is also a senior research fellow at the New School in New York City, it said.
Another Iranian-American, journalist Parnaz Azima, who works for the U.S.-funded Radio Farda, has been prohibited from leaving Iran since her passport was seized in January. She has been interrogated several times by intelligence agents since, the journalists rights group Reporters Without Borders said in a statement Wednesday.
Another American, former FBI agent Robert Levinson, disappeared in March after going to Iran's resort island of Kish, and his whereabouts are unknown.
Reporters Without Borders said a French-Iranian journalism student, Mehrnoushe Solouki, was arrested in February. She was released in March on bail but her passport was taken away and she has been unable to leave the country, it said.