Haleh Esfandiari was detained in Iran on May 8 and has been accused of "seeking to topple the ruling Islamic establishment."
State Department spokesman Sean McCormack said such charges were "utter nonsense."
He likened the case to that of Parnaz Azima, a reporter for U.S.-funded Radio Farda., who had her passport confiscated when entering Iran several months go and has not been allowed to leave.
"These are people [who] are interested in building bridges between the American and the Iranian people, and to talk about them in terms of a threat to the Iranian Government is just really nonsense," McCormack told a briefing.
"They are notable individuals in their fields of pursuit, whether it's journalism or in academia," he said. "So I can't tell you what the motivations are if the Iranian government -- indicating that they're going to be pursuing these charges. You'll have to ask the Iranian government. But any sort of hint or statement that these individuals were a threat to the Iranian government is just really poppycock."