TEHRAN, Iran - An Iranian official said Tuesday that the country had no plans to swap three American prisoners for a missing nuclear scientist who Tehran says was abducted by the United States.

Foreign Ministry spokesman Ramin Mehmanparast said it was not Iran's practice to "exchange people whose cases are still with the judiciary" and dismissed suggestions that the fate of the three Americans was linked to that of scientist Shahram Amiri.

"These two cases are not comparable," Mehmanparast told reporters in Tehran. "Iran will use legal channels to secure the release of Amiri."

Iran alleges the scientist, who disappeared while on a pilgrimage to Saudi Arabia, was abducted by the United States in 2009.

The three Americans - Sarah Shourd of California; her boyfriend, Shane Bauer of Minnesota; and their friend Josh Fattal of Elkins Park - were arrested along the Iraqi border in July. Iran has accused them of espionage and entering the country illegally. Their families say that the three were hiking in Iraq's largely peaceful mountainous northern Kurdish region and that if they crossed the border, it was accidental.

Iran allowed the mothers of the three to visit them in May, the families' first contact with them since they were taken into Iranian custody. The mothers had hoped to secure their children's release but returned empty-handed to the United States after a two-day visit.

Also in May, Iranian Intelligence Minister Heidar Moslehi signaled that Tehran might be open to a prisoner swap with the United States for the three Americans. President Mahmoud Ahmadinejad had also mentioned a swap in March, but nothing was officially proposed.

Before Amiri disappeared, he worked at Iran's Malek Ashtar University, an institution closely connected to the country's powerful Revolutionary Guard.

U.S. media reported in March that Amiri had defected to the United States and was assisting the CIA in efforts to undermine Iran's nuclear program.

On Tuesday, Iran summoned the Swiss ambassador to discuss Amiri. Switzerland represents U.S. interests in the absence of diplomatic relations between Washington and Tehran.