Iran has suggested that Washington propose a prisoner swap in return for the release of three Americans held in Teheran accused of spying.
The Americans say they entered Iran accidentally while tramping in northern Iraq last year.
Intelligence minister Heydar Moslehi said he had no doubt they were spies.
The Iranian news agency says he will not release them without a gesture from the United States.
Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, have been charged with espionage.
Their families said they were hiking and strayed over the border accidentally in July last year.
Tehran allowed the mothers of the three detainees to meet them last week, but ignored repeated pleas for their release.
The BBC reports the case has further strained relations between Washington and Tehran, which are at loggerheads over Iran's nuclear programme.
The US State Department said last Thursday that Washington was not contemplating any kind of a prisoner swap.
Iranian President Mahmoud Ahmadinejad suggested in an US TV interview last September that release of the trio might be linked to the freeing of Iranian diplomats he said were being held by US forces in Iraq.