Three American hikers who have been detained in Iran after crossing into the country from Iraq have been charged with espionage, Iran's State News Agency reported.

Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, have been detained in Iran since July 31. Their families say they entered the Islamic Republic accidentally while hiking in a scenic area of northern Iraq.

On Monday, Secretary of State Hillary Rodham Clinton called on Iran to release the three Americans, saying the U.S. believes "strongly that there is no evidence to support any charge whatsoever."

The three Americans 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.

Tehran's chief prosecutor 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 had been filed. In Iran's opaque judicial system, the process of charging and trial can often take place behind closed doors.

Clinton said the U.S. would continue to make that case through the Swiss channels who represent U.S. interests in Tehran. She spoke at a news conference in Berlin, where she is marking the 20-year anniversary of the fall of the Berlin Wall.

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

Bauer, Shourd and Fattal have been visited by Swiss diplomats, who oversee U.S. interests in Iran.

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."

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The Associated Press contributed to this report.