Haleh Esfandiari, director of the Middle East Program at the Washington-based Woodrow Wilson Center for Scholars, has been held for a week at Tehran's Evin Prison. She came to Iran months ago to visit her 93-year-old mother and was prevented from leaving.
The Iranian judiciary's announcement Tuesday was the first official word on an investigation into Esfandiari, who has been living in the United States since 1980.
Asked about Esfandiari's detention, Rice said she "ought to be released immediately."
"It just underscores the nature of the Iranian regime and it just gives strength to the argument that the regime does not, in addition to all of the problems that it causes internationally, does not treat its people ... very well," Rice told reporters in Moscow, where she was meeting with Russian leaders.
Over the weekend, the hard-line Iranian newspaper Kayhan said Esfandiari was accused of spying for the United States and Israel and had formed networks of activists to overthrow the Iranian government.
Her husband, Shaul Bakhash, called the accusations "fantasies" and "untrue."
Judiciary spokesman Ali Reza Jamshidi said Esfandiari was being investigated for "security" crimes, but did not give details or say what specific actions led to her arrest.
"Charges against Esfandiari are being investigated by the Intelligence Ministry ... She is being held at Evin prison," Jamshidi told reporters Tuesday.
The 67-year-old Esfandiari has for years brought prominent Iranians to Washington to talk about the political situation in Iran, some of whom have been subsequently detained and questioned back home. Her defenders say some of those she brought to the United States were supporters of the Iranian government who sought to explain Tehran's stance to Americans.
Esfandiari had been trapped in Iran since December, when three masked men with knives stole her luggage and passport as she headed to the airport to leave the country, the Wilson Center said. In the weeks before her arrest, she was called in for questioning daily on her activities, it said.
Her arrest came amid increasing restrictions on Iranian rights groups, particularly women's organizations, and other critics by the hard-line government of President Mahmoud Ahmadinejad. Iranian authorities have stepped up their warnings that the United States aims to use internal critics to destabilize the Iranian government amid the mounting tensions between the two countries.
Also Tuesday, Jamshidi said an Iranian journalist who was detained in December at Tehran's airport after returning from a conference in India has been sentenced to three years in prison. Jamshidi did not specify the charges against Ali Farahbakhsh, but the journalist's lawyer, Morteza Alizadeh Tabatabaei, said he was detained on charges of espionage.
Other Iranian-Americans have been prohibited from leaving Iran in recent months, including journalist Parnaz Azima, who works for the U.S.-funded Radio Farda. Another American, former FBI agent Robert Levinson, disappeared in March after going to Iran's resort island of Kish.