Kian Tajbakhsh, an urban planning consultant who has also worked for the World Bank, was detained around May 11, according to George Soros' Open Society Institute. The group is a private foundation that encourages democracy-building in countries around the world.

His detention, and the detention of other Iranian-Americans in Iran, come amid recent accusations by Iranian authorities that the U.S. is using critics and dissidents to try to overthrow the country's hard-line government.

"We have no exact report about him from official authorities," said Mohammad Ali Hosseini, spokesman of Iran's Foreign Ministry, when he was asked about Tajbakhsh, who is also a research fellow at the New School in New York.

The Open Society Institute has called for his immediate release. It said he 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."

The president of the New School, Bob Kerrey, said Tajbakhsh's detention was "unprovoked and without justification."

Hosseini also said he had no information about another Iranian-American, Ali Shakeri, who was supposed to leave Iran and fly to Europe on May 13 but never arrived at his destination.

"About Mr. Shakeri, we have to ask authorities for information," he said.

Friends and colleagues of Shakeri, a founding board member at the University of California, Irvine's Center for Citizen Peacebuilding, worry that he has been taken into custody in Iran following a trip there to visit his ailing mother.

Since the U.S.-led invasion of Iraq, Iran has often accused the United States and Britain of trying to undermine its security.

The recent detentions come as tensions have mounted between the United States and Iran, even as the two countries prepare to hold ambassador-level talks scheduled for Monday in Baghdad on calming Iraq's violence.

Earlier this month, Iranian authorities arrested Haleh Esfandiari, director of the Middle East Program at the Washington-based Woodrow Wilson Center for Scholars. The government accuses Esfandiari, also an Iranian-American, of working for an organization that it claims was seeking to topple the government. Both Esfandiari's family and the Wilson Center deny the allegations.

Another Iranian-American, Parnaz Azima, a journalist for the U.S.-funded Radio Farda, has been prohibited from leaving Iran since her passport was seized in January.

Robert Levinson, a former FBI agent, disappeared in March after going to Iran's resort island of Kish.