Tehran prosecutor said on Monday that three U.S. citizens detained by Iranian security forces on the Iran-Iraq border in late July are charged with espionage, the official IRNA news agency reported.

"The three are charged with espionage and the investigation into them is underway. The comment about them will be announced in the near future," said Abbas Jafari Dolatabadi.

Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, were taken into custody by Iranian border security forces on July 31.

Earlier, the U.S. Secretary of State Hillary Clinton called on the Iranian officials to release the detained U.S. citizens.

On Sunday, friends and relatives of the detained Americans kept vigils across the United States to call for the release of them.

Swiss ambassador to Tehran who acts on behalf of the U.S. interests in Iran has already visited the detained Americans in Iran's prison.

The U.S. broke off diplomatic relations with Iran in 1980 and their ties have remained severed ever since.