TEHRAN, Iran — Iran accused three detained Americans of spying Monday, signaling Tehran intends to put them on trial. It drew a sharp U.S. response that the charges are baseless because the hikers strayed across the border from Iraq.

The announcement comes as Washington and Tehran are deadlocked in negotiations over Iran's nuclear program, raising concern that the three could be used as bargaining chips in the talks or to seek the return of Iranians they say are missing.

Relatives and the U.S. government say the three were innocent tourists on an adventure hike in northern Iraq and accidentally crossed into Iran where they were arrested on July 31.

Commenting on the case, President Mahmoud Ahmadinejad accused the U.S. of jailing innocent Iranians and pointed to two of his countrymen — a nuclear scientist and a top defense official — who disappeared in recent years. Tehran accuses the U.S. of kidnapping them. The U.S. has refused comment on the two, and there has been speculation they defected to the West.

Ahmadinejad, asked about the spying accusations against the Americans, told reporters in Istanbul, Turkey, said he had no opinion about the case.

"It must be judged by the judiciary, whether they are spies or not," he said. "There are some Iranians who have spent many years in prison without doing anything wrong, in American prisons."

He said the Americans had crossed the border illegally and Iran has a right to punish them.

"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."

The Americans — Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27 — have been held in Iran's Evin prison, where Swiss diplomats have visited them twice and said they are healthy.

The three graduates of the University of California at Berkeley had been trekking in Iraq's northern Kurdistan region, their relatives say.

The case recalled that of an American-Iranian journalist, Roxanna Saberi, who was arrested in Iran in January and convicted of espionage. After heavy pressure from the U.S., she was freed on appeal in May and returned home — and several months later, the U.S. military released five Iranians it had held for more than two years.

The accusations against the three Americans could be a first step in a similar move by Iran to put them on trial and convict them, then arrange their release, aiming to get concessions.

Bauer and Shourd had been living in Damascus — he studying Arabic, she teaching English — and both had done freelance journalism or writing online. Friends described them as passionate adventurers interested in the Middle East and human rights.

Fattal, who spent three years with a group dedicated to sustainable farming near Cottage Grove, Ore., had been overseas since January as a teaching assistant with the International Honors Program.