Iran's foreign ministry has confirmed that the government has detained a leading Iranian-American academic.
Haleh Esfandiari is one of the leading US authorities on Iran
Spokesman Mohammad Ali Hosseini said the arrest of Haleh Esfandiari was lawful and she would be treated like other Iranian nationals.
Ms Esfandiari, one of Washington's best known Iran experts, was visiting Tehran to see her 93-year-old mother.
The incident comes at a time of continuing tension between the United States and Iran.
"It is natural if there is any problem, it will be handled by authorities," said the spokesman.
Iran does not recognise dual nationalities.
In December, as she was on her way to the airport to return to the US, Ms Esfandiari's taxi was stopped by three men who stole her belongings, including her Iranian and US passports.
When she went to replace her passport, she was sent to the intelligence ministry, where she was repeatedly questioned about her work as the director of the Middle East Program at the Woodrow Wilson Centre in Washington.
Last week, after being prevented from leaving the country for more than four months, she was taken to the notorious Evin Prison in Tehran by three masked men armed with knives, the Woodrow Wilson Centre said.
Iran's Kayhan newspaper has accused Ms Esfandiari of spying for the US and Israel and of trying to incite a democratic revolution in the country.
Her husband, Shaul Bakhash, denied the newspaper's allegations.
"It is a false and hollow accusation that Haleh Esfandiari is one of the 'principle instruments' of Israel, or a Mossad spy service, in advancing the strategy of a 'velvet revolution' in Iran," he said in a statement sent to Associated Press news agency.
"It is a lie that Haleh Esfandiari had 'undercover assignments' or that she was one of the 'media spies' in Iran."
Iranian authorities appear to be particularly suspicious of attempts by the Bush administration to promote democratic change in Iran, says the BBC's Middle East analyst, Roger Hardy.
Other Iranian-Americans have also been banned from leaving the country recently, including journalist Parnaz Azima, who works for the US-funded Radio Farda.
Former FBI agent Robert Levinson disappeared in March on Iran's resort island of Kish.