Libyan rebels are threatening to isolate the capital, Tripoli, as they capture key towns to the south and west of the city and work to cut off supply routes in their bid to push leader Moammar Gadhafi from power. The rebels say they control most of Zawiya, a strategic town 50 kilometers west of Gadhafi\'s power base in Tripoli. Rebel fighters entered Zawiya Saturday in their closest approach to the capital since government forces crushed Zawiya\'s rebel movement in the early weeks of the uprising. Pro-Gadhafi forces exchanged fire with rebel fighters in Zawiya Monday, trying to push them back from the town center. Rebel spokesmen said their fighters also captured the towns of Surman, 60 kilometers west of Tripoli, and Gharyan, 80 kilometers south of the capital. Their claims could not be independently verified.