"The crime they committed is of illegally entering Iranian territory. The other things are at the level of accusations," Foreign Minister Manouchehr Mottaki told reporters, indicating that the trio may not have been formally charged with spying as reported on Monday.
"The judiciary is examining their case… but what is important is the verdict which will be pronounced against them," Mottaki said.
Iranian forces in July captured Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, near the border with Iraq.
On Monday, Tehran's chief prosecutor Abbas Jaffari Doulatabadi said investigations were continuing.
"The three Americans arrested near the border of Iran and Iraq are facing accusations of spying and the inquiry is continuing," he was quoted by the official IRNA news agency as saying.
Washington has repeatedly called for their swift release, saying they are innocent hikers and were wrongly detained.
US Secretary of State Hillary Clinton said there was no evidence for Iran to charge them.
"We believe strongly that there is no evidence to support any charge whatsoever," she told reporters in Germany.Related article: US urges release of hikers
"And we would renew our request on the behalf of these three young people and their families that the Iranian government exercise compassion and release them so they can return home."
However, Iranian President Mahmoud Ahmadinejad insisted that justice must run its course.
"There are rules in every country that punish illegal entry to its territory. It is an obligation to respond to such an event," he told a news conference in Istanbul.
Asked about possible espionage charges, he said that was entirely a matter for the courts.
"I have no opinion," he said. "It is the judges who know whether they are spies or not."
Family and friends of the three have said they unintentionally strayed into Iran from Iraq.
