US Secretary of State Hillary Clinton has called on Tehran to free three US citizens who crossed into Iran while hiking in Iraq.
She was speaking after Iranian Foreign Minister Manouchehr Mottaki said the three would stand trial, although he did not specify on which charges.
Mrs Clinton said that any charges would be unfounded.
Shane Bauer, Sarah Shourd and Josh Fattal were detained in July and have been accused of illegal entry.
Iranian officials have also previously alleged the trio were spying.
Their families say they strayed across the border accidentally.
Mrs Clinton said: "We appeal to the Iranian leadership to release these three young people and free them as soon as possible.
"We consider this a totally unfounded charge. There is no basis for it... They were out hiking and unfortunately, apparently, allegedly walked across an unmarked boundary."
Mr Mottaki earlier told a news conference that "interrogation of the three Americans who have illegally entered Iran with suspicious aims is ongoing".
"They will be put on trial by the judiciary and rulings will be made," he said in comments broadcast on state television.
HIGH PROFILE DETENTIONS
25 November: Five Britons arrested after their racing yacht strays into Iranian waters. Freed a week later.
Jan 09: US-Iranian journalist Roxana Saberi arrested for buying a bottle of wine. Jailed for spying in April, but released a month later after an appeal.
23 March 07: 15 Royal Navy personnel seized by Iranian warships after searching a cargo boat in the Gulf. Held for nearly two weeks.
The three Americans were seized by Iranian border guards on 31 July near the town of Marivan, and are being held in Tehran's Evin prison.
Swiss diplomats - who represent US interests in Iran, with whom the US has no formal diplomatic relations - were allowed to meet the trio in late September for the first time since their arrest.
Their families say the spying allegations are "entirely at odds with the people Shane, Sarah and Josh are and with anything that Iran can have learned about them since they were detained".
The BBC's Tehran correspondent, Jon Leyne, in London, says Iran's latest move could be seen by Washington as a deliberate provocation.
The three hikers are all friends who attended the prestigious University of California, Berkeley.
Shane Bauer, 27, is a freelance journalist and photographer, who specialises in reporting on the Middle East and the Darfur region of Sudan. He lives in Damascus with Sarah Shourd, 31, a teacher and writer.
Josh Fattal, 27, an environmentalist and teacher, had travelled to Damascus in the summer to join them for the hiking holiday.
He reportedly posted a Facebook entry on 17 July, saying that he was in the Middle East and planning a trip to Kurdistan.