TEHRAN, Iran -- A senior Iranian prosecutor accused three Americans detained on the border with Iraq of espionage on Monday, the first signal that Tehran intends to put them on trial.

The action 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 were maneuvering over a deadlock in negotiations over Iran's nuclear program.

President Mahmoud Ahmadinejad alleged the three crossed Iran's border illegally, saying this was something any country would punish. Asked in Istanbul if he thought they were spies, he said:

"I have nothing to say about it. I have no opinion about it. It must be judged by the judiciary, whether they are spies or not. There are some Iranians who have spent many years in prison without doing anything wrong, in American prisons."

Ahmadinejad spoke at a news conference before the start of a summit of the 57-nation Organization of the Islamic Conference.

"In all countries, crossing borders would have a very heavy sentence, according to the law," he said. "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."

White House Press Secretary Robert Gibbs said the Americans were innocent hikers and called for their release. U.S. Secretary of State Hillary Rodham Clinton said the spying charges were baseless.

"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.

Swedish Prime Minister Fredrik Reinfeldt, whose country holds the rotating European Union presidency, warned Iran against using the three for political leverage.

"The only thing I can say is that I hope that we don't use the lives of very young people for political purposes," Reinfeldt said.

Shane Bauer, whose sister lives in Boulder, Colo., 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 U.S. 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.

About two dozen people gathered for a vigil for the missing hikers on Sunday in Boulder, where the Bauer's sister lives. Many were wearing t-shirts that said, "Free The Hikers."

"We know they had no intention of crossing the border," Shannon Bauer said. "We think 100 days is punishment enough for a mistake."

According to his Web site, Shane Bauer is a freelance journalist and photographer based in the Middle East. A fluent speaker of Arabic, his work has largely focused on the Middle East and North Africa, where he has spent much of the past six years. He is a Middle East correspondent for New America Media and his work has been published in the U.S., U.K., Middle East, and Canada including media outlets such as the L.A. Times, San Francisco Chronicle, Christian Science Monitor, The Nation, Le Monde Diplomatique (German edition), Slate.com, Aljazeera.net, The San Francisco Bay Guardian, Democracy Now!, E: The Environmental Magazine, and Black Entertainment Television.

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.