Updated

Iran said Sunday it had "no exact report" about an Iranian-American employee of a U.S. foundation promoting democracy missing after a visit to his homeland — one of several people believed detained in Iran amid rising tensions with the United States.

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.

Hosseini also said he did not have any information 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.

When it announced the charges against Esfandiari, the Iranian Intelligence Ministry accused the Open Society Institute of being involved in the plot to set up the network to overthrow the government.

The Open Society Institute has said 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." The institute said it was concerned for his safety and called for his immediate release.

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.

Reporters Without Borders also has 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.