The move could set up the Americans - who relatives say were hiking and strayed across the border from Iraq - as potential bargaining chips in Iran's standoff with the West. The announcement came as Washington and Tehran are maneuvering over a deadlock in negotiations over Iran's nuclear program.

Among those facing charges is Josh Fattal, an Elkins Park native.

Iranian President Mahmoud Ahmadinejad alleged the three crossed Iran's border illegally, saying this was something any country would punish.

"In all countries, crossing borders would have a very heavy sentence, according to the law," he told a news conference in Istanbul before the start of a summit of the 57-nation Organization of the Islamic Conference. "Unfortunately, they crossed our borders illegally. We are not happy about that, but there is a law."

"Hopefully, they will have an appropriate answer in the court, and hopefully they will convince the judge that they did not have any intention of crossing the border illegally," Ahmadinejad said.

U.S. Secretary of State Hillary Rodham Clinton called for the release of the three. "We believe strongly that there is no evidence to support any charge whatsoever," she 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 U.S. would continue to make that case through the Swiss channels who represent U.S. interests in Tehran.

Fattal, Shane Bauer and Sarah Shourd - all graduates of the University of California, Berkeley - were arrested July 31 after straying over the Iranian border from northern Iraq. The U.S. government and their families say there were on a hiking vacation and crossed accidentally.

Bauer's sister, Shannon Bauer, who lives in Boulder, Colo., declined to comment Monday, a day after she organized a candlelight vigil for the hikers there. It marked the 100th day since they had been detained.

Bauer is originally from Onamia, Minn. and had been living in Damascus, Syria, with Shourd, his girlfriend.

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 has seemed in the past to use jailed Americans as a bargaining chip.

In January, Iranian-American journalist Roxana Saberi was arrested in Tehran, was convicted of espionage, then released on appeal in May. Two months later, U.S. forces in Iraq freed five Iranians who they had been holding for months.

Iran is also currently 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.

A Canadian-Iranian reporter for Newsweek, Maziar Bahari, was a defendant in the same mass trial over the protests, which erupted after opposition charges that June presidential elections were fraudulent. Bahari was released on bail last month and left Iran, joining his pregnant wife in London.