ZINTAN, Libya—Rebel fighters have penetrated Libya's southwest desert and pulled within 80 miles of Col. Moammar Gadhafi's southern stronghold, opening a new front and suggesting the strongman's grip is slipping even in areas believed firmly in his control.

The rebels captured a small village south of Sebha on Monday. The fall of Sebha, one of Col. Gadhafi's three regional power centers, would be a huge symbolic and strategic blow.

The...