TEHRAN, Iran (AP) - 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, 27, Sarah Shourd, 31, and Josh Fattal, 27, all graduated from the University of California, Berkeley. They were arrested July 31 after straying over the Iranian border from northern Iraq. The U.S. government and their families say there were hiking and crossed accidentally.

Tehran chief prosecutor Abbas Jafari Dowlatabadi says the three "have been accused of espionage" and that investigations were continuing, according to state news agency IRNA. He said an "opinion (on their case) will be given in the not distant future."

It was 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.

Ahmadinejad raised at the news conference several specific cases of Iranians he alleged were innocent, but held by the Americans.

"Recently an Iranian went to Mecca for pilgrimage and he was kidnapped by the American forces," he said. "And two years ago, an Iranian was also kidnapped by Americans in Turkey and their families are very much concerned about the fate of these people. They did not cross the American borders illegally. They were arrested in their own homes and their neighbors' homes."

Last month, Iran accused the United States of involvement in the mysterious disappearance in June of an Iranian nuclear scientist on a pilgrimage to the holy city of Mecca in Saudi Arabia. Shahram Amiri reportedly worked at a university linked to the elite Revolutionary Guard military corps and his disappearance raised questions about whether he defected and gave the West information on Iran's nuclear program.

In early 2007, Iran said its former deputy defense minister had gone missing while on a private trip to neighboring Turkey, and its top police chief accused Western intelligence services of possibly kidnapping the official. Ali Reza Asghari, a retired general in the Revolutionary Guard, had arrived in Turkey on a private visit from Damascus, Syria, according to Iranian state media.

Ahmadinejad had told The Associated Press in an interview in September he would ask the judiciary for leniency in the case.

"What I can ask is that the judiciary expedites the process and gives it its full attention, and to basically take a look at the case with maximum leniency," he said.

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 appeared in the past to use jailed Americans as bargaining chips.

In January, Iranian-American journalist Roxana Saberi was arrested in Tehran, 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 two and a half years.

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.