Iran charged three American adventure tourists with spying today after they were caught straying across the border from Iraq.
The move was the first signal that Tehran intends to put Shane Bauer, Sarah Shourd and Josh Fattal on trial.
They were held on July 31 during what the US government and their families say was on a hiking holiday on which they crossed the border accidentally.
US Secretary of State Hillary Clinton called for the release of the three. The announcement came as Washington and Tehran are manoeuvring over a deadlock in negotiations over Iran's nuclear program.
"We believe strongly that there is no evidence to support any charge whatsoever," Mrs Clinton said.
"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."
Tehran chief prosecutor Abbas Jafari Dowlatabadi said the three "have been accused of espionage" and that investigations were continuing. 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 programme. Iran is also holding another American, academic Kian Tajbakhsh, who was arrested amid Iran's post-election 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 they had been holding for months.