VIENNA - A senior US official expressed concern on Friday about a lack of progress in negotiations between six world powers and Iran over Tehran's disputed nuclear program, speaking on the expected last day of a May 13-16 meeting."Talks have been slow and difficult," the official said, declining to be named. "Significant gaps remain. Iran still has some hard decisions to make. We're concerned that progress is not being made and that time is short."This week's talks are the fourth round of negotiations between Iran and the powers - the United States, France, Germany, Britain, China and Russia - aimed at reaching a long-term deal by a self-imposed July 20 deadline.