Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, were detained after they entered Iran from northern Iraq, further complicating relations between Tehran and Washington that are deadlocked over Iran's nuclear programme.
Their families said they were hiking and had strayed across the border accidentally. The United States, which has not had diplomatic ties with Iran since shortly after the 1979 Islamic revolution, has called for their release.
"Tehran has compelling evidence that three Americans arrested in Iran last year were cooperating with intelligence services," Intelligence Minister Haidar Moslehi was quoted as saying by English language website Press TV.
Moslehi gave no further details, adding only: "Iran will present its evidence to the media soon." Under Iran's law, espionage can be punishable by death.
