US Secretary of State Hillary Rodham Clinton called for the release of the three. The announcement came as Washington and Tehran are maneuvering over a deadlock in negotiations over Iran's nuclear program.
"We believe strongly that there is no evidence to support any charge whatsoever," Clinton told reporters in Berlin. "And we would renew our request on behalf of these three young people and their families that the Iranian government exercise compassion and release them, so they can return home."
Clinton said the US would continue to make that case through the Swiss channels who represent US interests in Tehran.
Shane Bauer, Sarah Shourd and Josh Fattal, all graduates of the University of California, Berkeley, were arrested July 31 after straying over the Iranian border from northern Iraq. The US government and their families say there were on a hiking vacation and crossed accidentally.
Tehran chief prosecutor Abbas Jafari Dowlatabadi says the three "have been accused of espionage" and that investigations were continuing, according to the state news agency IRNA. He said an "opinion (on their case) will be given in the not distant future."
It is not clear from his comments whether formal charges had been made, but such announcements are often a sign that charges are imminent if not already filed. In Iran's opaque judicial system, the process of indictment and trial often takes place behind closed doors.
The timing of the announcement raised the possibility that Iran was using the case to pressure the United States amid the negotiations over its nuclear program.
Iran is also holding another American, academic Kian Tajbakhsh, who was arrested amid Iran's postelection turmoil and was sentenced last month to 12 years in prison for an alleged role in opposition protests.
In January, Iranian-American journalist Roxana Saberi was arrested in Tehran, was convicted of espionage, then released on appeal in May. Two months later, US forces in Iraq freed five Iranians who they had been holding for months.