TOBRUK, Libya — Moammar Gadhafi's forces have launched an assault on a rebel-held town west of Tripoli near the Tunisian border.

Residents say government troops shelled the town of Zwara with tanks and artillery and were blocking its entrances. One rebel supporter in the town said at least four opposition fighters were killed. The residents spoke on condition of anonymity for fear of retaliation.

The assault was the latest attempt by the regime to clear rebels out of the northwestern corner of Libya around the capital. Last week, government forces took Zawiya, the closest opposition-held city to Tripoli.

Zwara lies 70 miles (110 kilometers) west of Tripoli, near the Tunisian border.