Opposition fighters in Libya have seized strategic positions outside the capital city of Tripoli, intensifying pressure on the regime of Col. Muammar Gaddafi. The rebels have cut off the major oil and diesel fuel pipelines to Tripoli and captured two key outlying towns. In response, Gaddafi’s troops shelled the coastal town of Az-Zawiyah, which rebels now control. A senior Libyan security official unexpectedly arrived in Cairo Monday, signifying a major defection for the Gaddafi regime. U.S. officials, meanwhile, say Gaddafi’s forces have fired a surface-to-air scud missile for the first time in the ongoing conflict, but it failed to hit its target. The developments inside Libya come as representatives of the Gaddafi regime and the rebels continue to hold talks in neighboring Tunis. The United Nations envoy on Libya — former Jordanian foreign minister Abdul Ilah al-Khatib — flew into Tunis on Monday to join the negotiations.