Three American hikers who have been detained in Iran since they crossed into the country from Iraq in July have been accused of espionage, the first signal that Tehran intends to send the matter to trial.

Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, entered the Islamic Republic while hiking along the unmarked border in the scenic mountains of Iraqi Kurdistan, a peaceful region that is becoming popular among Western tourists. Their families and the U.S. government say they crossed into Iran accidentally.

Tehran chief prosecutor Abbas Jafari Dowlatabadi said Monday the three "have been accused" and that investigations were continuing, according to the state news agency IRNA. He said an "opinion" on their case will come "in the not distant future."

It is not clear from his comments whether formal charges have been filed. In Iran's judicial system, the process of charging and trying suspects often takes place behind closed doors.

The hikers' families say the allegations are untrue and that their loved ones should be freed.

In a group statement released Monday, the families said the accusations are "entirely at odds" with the kind of people Shane Bauer, Sarah Shourd and Josh Fattal are.

The Iranian government arrested the Americans on July 31 while hiking near the Iraqi border.

The families asked the Iranian government to show compassion and release the three without delay.

Secretary of State Hillary Clinton called on Iran to release the three Americans on Monday.

"We believe strongly that there is no evidence to support any charge whatsoever," Clinton told reporters in Berlin, where she is marking the 20-year anniversary of the fall of the Berlin Wall. "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."

The three Americans — all of whom graduated from 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.

SLIDESHOW: American Hikers Detained in Iran

Clinton said the U.S. would continue to make that case through the Swiss channels that represent U.S. interests in Tehran. Bauer, Shourd and Fattal have been visited by Swiss diplomats, who oversee U.S. interests in Iran.

The families of the hikers, who have had no contact with them since they were detained, released videos last month that showed them dancing and singing before they were captured by Iranian authorities.

Bauer's uncle, Kenneth Bauer, of Shakopee, Minn., called on Iranian authorities to immediately release his nephew, whom he described as a "happy-go-lucky" man who likes to help people in need.

"That's why he's down there," Bauer told FoxNews.com on Monday. "He just works enough for food and helps people out. God, we miss him."

Asked what message he would pass on to his nephew, Bauer replied, "You've got to try to keep your spirits up."

Iranian President Mahmoud Ahmadinejad said in an interview with The Associated Press in September that he could ask the judiciary to "take a look at the case with maximum leniency."

In a statement obtained by FoxNews.com on Monday, relatives of the hikers said the allegations were untrue.

"It is 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 on July 31," the statement read. "Shane, Sarah and Josh have now been held for more than 100 days simply because they apparently strayed into Iran by accident while hiking in Iraqi Kurdistan. We again call on Iran to show compassion to our loved ones and release them without delay. This has already gone on for too long.”

FoxNews.com's Joshua Rhett Miller and The Associated Press contributed to this report.