President George W. Bush said today he strongly condemned Iran's detention of US citizens and called for them to be freed "immediately and unconditionally."
"Their presence in Iran - to visit their parents or to conduct humanitarian work - poses no threat," Mr Bush said in a statement that referred to Haleh Esfandiari, Parnaz Azima, Kian Tajbakhsh and Ali Shakeri.
Mr Bush also demanded to know the whereabouts of Robert Levinson, who has been missing since early March.
Earlier the US State Department said Iran has detained a fourth Iranian-American.
The State Department said it believed California businessman Ali Shakeri had been detained about 10 days ago and was being held in Tehran's Evin prison.
It said the arrest was part of a "disturbing pattern" of harassment of dual citizens.
Mr Shakeri is the fourth Iranian American to be detained, imprisoned or otherwise barred from leaving Iran in recent months in a series of incidents that US diplomats fear may herald a further deterioration in US-Iranian relations.
The other three - academic Haleh Esfandiari, social scientist Kian Tajbakhsh and journalist Parnaz Azima - have been charged with spying, an offence that could carry the death sentence under Iran's sharia law.
Washington has denied the three are spies and demanded their release, saying they are private citizens who went Iran to meet family and have ordinary professional contacts.
Tehran accuses Washington of using intellectuals and others inside the country to undermine the Islamic Republic through what it calls a "velvet revolution".