Relations between the United States and the Islamic Republic are strained by what Western powers believe are Iranian efforts to develop a nuclear weapon. Iran denies the accusation, but US-led efforts are afoot to impose new sanctions on Tehran.

The US State Department has called for the freeing of the three, who entered Iran from northern Iraq. Iran's judiciary has laid espionage charges against Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27.

Their families said they were hiking and strayed over the border accidentally.

"Their status as spies is explicit and certain and there is no equivocation in regard to a swap," Heydar Moslehi told reporters on the sidelines of an Iranian Cabinet meeting, the semi-official Fars news agency reported.

"Our expectation is that the Americans, with their claim on human rights issues, should initiate an action so that we can decide on whether or not there would be one (a swap)," he said.