Speaking in Washington after a meeting with Spanish Foreign Minister Miguel Angel Moratinos, Clinton said spying charges brought against the three by Tehran are "totally unfounded."
Clinton asserted the hikers "have absolutely no connection with any kind of action against the Iranian state or government," and added, "We appeal to the Iranian leadership to release these three young people and free them as soon as possible."
Her remarks came hours after Iranian Foreign Minister Manouchehr Mottaki announced the three hikers, arrested four months ago, will stand trial on charges of spying.
The three were detained near Iran's border with Iraq, after they "illegally" entered Iran, the Iranian Labor News Agency reported.
"Interrogation of the three Americans who have illegally entered Iran with suspicious aims is ongoing," ILNA quoted Mottaki as saying. "They will be put on trial by the judiciary and rulings will be made."
In late November, the Tehran prosecutor said authorities were pursuing espionage charges against the Americans -- Shane M. Bauer, 27; Joshua F. Fattal, 27, and Sarah E. Shourd, 31. U.S. officials, including White House and State Department officials -- repeatedly urged Iran to release the hikers, saying they got off-course and strayed into Iran from Iraq's Kurdish-speaking region.
Statements made by family members and Kurdish authorities said the hikers entered Kurdistan from Turkey, The New York Times reported.