TEHRAN (Reuters) - Iran has charged three detained U.S. citizens with espionage, the official IRNA news agency quoted a judiciary official as saying on Monday.

The three were held after they strayed into Iran from northern Iraq at the end of July.

"We believe strongly that there is no evidence to support any charge whatsoever," U.S. Secretary of state Hillary Clinton said on a visit to Berlin on Monday.

The three, Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, crossed into Iran from Iraq and their families say they strayed across the border accidentally.

"The three are charged with espionage. Investigations continue into the three detained Americans in Iran," Tehran general prosecutor Abbas Jafari Dolatabadi told IRNA.

Iranian President Mahmoud Ahmadinejad suggested in an interview with the American television network NBC in September that the Americans' release might be linked to the release of Iranian diplomats he said were being held by U.S. troops in Iraq.

Under Iran's Islamic sharia law, espionage is punishable by death.