"As of our latest information, the plane has not departed Iran," the official said. "Having concluded this agreement, there were a number of logistical steps that have to take place."
"We expect those to be completed as soon as possible."
Four Americans -- Washington Post journalist Jason Rezaian; Saeed Abedini, a Christian pastor from Idaho; Amir Hekmati, a former US Marine; and a fourth man named Nosratollah Khosravi -- are to be released in exchange for seven Iranians currently held in the United States.