Ramin Mehmanparast said on Tuesday it's not Iran's practice to "exchange people whose cases are still with the judiciary."
He says the fate of the trio is not linked to that of Iranian nuclear scientist Sharam Amiri, who Tehran claims was abducted by the U.S. in 2009.
The three Americans - Sarah Shourd, her boyfriend, Shane Bauer of Minnesota, and their friend Josh Fattal - were arrested along the Iraqi border last July. Their families say they were hiking in northern Iraq's Kurdish region and if they strayed into Iran, it was unintentional.
Iran allowed mothers of the three to visit them in May.
