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, 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.