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

U.S. Secretary of State Hillary Rodham Clinton called for the release of the three. The announcement came as Washington and Tehran are maneuvering over a deadlock in negotiations over Iran's nuclear program.

"We believe strongly that there is no evidence to support any charge whatsoever," Clinton 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.

Shane Bauer, 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.