The United States Wednesday called on Iran to release three American women - all of whom also hold Iranian citizenship - who have been barred from leaving that country after visits there. They include a prominent Washington-based academic, and a Radio Farda journalist. VOA's David Gollust reports from the State Department.

Officials here say it is unclear why the Iranian authorities are detaining the three women, who, they insist, pose no threat to that country and should be released as soon possible.

All three are understood to have been barred from leaving Iran, and subjected to questioning from authorities, after arriving in the country for family visits in recent months.

Colleagues of one of the women, Haleh Esfandiari, director of the Middle East Program at Washington's Woodrow Wilson International Center for Scholars, announced Tuesday she had been jailed at Tehran's Evin prison following weeks of interrogation focusing on her work.

Iran has similarly confiscated the passport of Parnaz Azima, a reporter based in Prague for the U.S.-funded Persian-language broadcaster Radio Farda.

Like Esfandiari, Azima went to Iran to visit her ailing mother a few months ago and has been prevented from leaving by officials who have said she may be detained for two or three years.

The case of the third woman is understood to be similar, though the State Department has not identified her at the request of her family.

At a news briefing, State Department Spokesman Sean McCormack urged the Iranian government to free the women on humanitarian grounds, and not stifle what unofficial contacts that remain between the two countries, which have not had diplomatic relations since 1979:

"I can't tell you the motivations behind these actions by the Iranian government," said Sean McCormack. "I can only repeat that these are people who don't pose any threat to the Iranian government, and in fact symbolize the kind of people to people interaction that we want to encourage. Whatever problems we may have with the policies of the Iranian government, we don't want to put a chill in those people-to-people contacts."

In another development, McCormack said State Department officials met Wednesday with the wife of Robert Levinson, a retired U.S. Federal Bureau of Investigation agent who went missing several weeks ago after arriving on Kish Island at a Persian Gulf resort.

Iran has denied, in notes to Washington passed through the Swiss government, that it has any knowledge of Levinson's whereabouts.

But U.S. officials are skeptical in light of press reports of his apparent arrest on the island, and have asked three other countries with diplomats in Iran to make inquiries on Levinson's behalf.

Levinson's wife Christine made a public appeal for help in locating her husband Wednesday on an Internet website the family has set up.

She said she sent a letter two weeks ago to Iranian President Mahmoud Ahmadinejad urging him to intercede in the case, but was unsure whether he has received it. The letter was carried in the website in both English and Farsi.