Three American hikers detained in Iran are to be charged with espionage, three months after they were arrested for crossing over the border from Iraq.

The spying allegation, revealed by Tehran’s chief prosecutor yesterday, is the first sign of Iran’s intention to put the three on trial, despite repeated demands from Washington for their release.

Sarah Shourd, 31, Josh Fattal, 27 and Shane Bauer, 27, were hiking near Ahmed Awa waterfall in the mountains of Iraqi Kurdistan on July 31 when they apparently strayed over the border into Iran.

They had travelled there from Damascus, the Syrian capital, where Mr Bauer, an Arabic-speaking freelance journalist, was based. The alarm was raised by a companion, Shon Meckfessel, who had stayed behind at their hotel in Sulemaniya recuperating from a cold.

Abbas Jafari Dowlatabadi, the prosecutor, told the state news agency that the three “have been accused of espionage” and that investigations into their case were continuing. He said an “opinion will be given in the not distant future,” a signal from Iran’s opaque judicial system that charges were imminent if not in fact already laid.

Hillary Clinton, in Germany for the celebrations marking the anniversary of the fall of the Berlin Wall, demanded that Tehran release the three immediately and without charge.

“We believe strongly that there is no evidence to support any charge whatsoever,” she told reporters. “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.”

Tehran’s announcement comes as yet another test of President Obama’s policy of engagement to persuade Iran to open up over a suspected military dimension to its nuclear programme. Despite a verbal agreement reached in Geneva last month, Iran continues to hedge on a key confidence-building deal to ship out stocks of nuclear fuel for reprocessing abroad.

The timing raised suspicions that Iran could be using the case to pressure the United States into accepting a counter demand that leaves it in possession of most of its stockpile of nuclear fuel despite fears that it could use it for a weapons program. Western powers fear a repeat of the North Korean scenario in which Pyongyang enriched uranium under United Nations supervision, only to kick out the inspectors and rev up its programme as soon as it had acquired enough fuel to build a bomb.

Laura Ling and Euna Lee, two American journalists arrested earlier this year after straying into North Korea, were released only after the personal intervention of former president Bill Clinton who flew to Pyongyang to collect them - a move denounced by critics as bowing to the rogue state’s demands. North Korea has since demanded bilateral talks with Washington if it is to agree to resuming nuclear negotiations within the previous six-party structure.

Iran is currently holding two other Americans, both dual Iranian-American nationals, including Kian Tajbakhsh, an academic arrested amid the June post-election turmoil and sentenced last month to 12 years in prison for his alleged role in opposition protests.

Roxana Saberi was arrested in Tehran in January and convicted of espionage, then released in May after appealing her eight-year sentence.

Her release was followed two months later with the freeing of five suspected Iranian agents held by the United States military in Iraq.

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