Hundreds of rebels have launched a broad offensive against government forces in western Libya and seized three small towns, spokesmen say.
Four rebel fighters were killed and several wounded in what the spokesmen described as the biggest push in the area since the start of Libya's civil war five months ago.
They said rebels captured 18 government soldiers, as well as weapons and ammunition.
Libya's civil war has been largely deadlocked and rebels have been unable to score a major breakthrough on the battle field.
Rebels control Libya's eastern third and most of the Nafusa mountains in the west, as well as the western port city of Misrata. Libyan leader Muammar Gaddafi, entrenched in the capital of Tripoli, is clinging to the rest of the territory.
In Thursday's fighting, hundreds of rebels descended from their stronghold in the Nafusa mountains at dawn and advanced along a stretch of more than 60 miles.
Opposition forces drove government troops out of the small towns of Jawsh, Ghezaya and Takut, the rebels said.
Jawsh is about 100 miles east of the Tunisian border and along a main road between Tunisia and Tripoli while Ghezaya is closer to the Tunisian border, and had served as a base for shelling attacks by Gaddafi's forces on the rebel-held border town of Nalut. Rebels were also advancing toward the town of Badr and two other areas.
One of the objectives of the push is to drive back pro-Gaddafi forces from the rebels' main supply route between the Tunisian border and the Nafusa mountains.
In a separate development, the rebels said they had detained their own top military commander for questioning on suspicion his family might still have ties to the regime.