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 Mr. 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.

Control of Zawiya, Surman and Gharyan would allow the rebels to cut off Tripoli from a key highway to the south and another leading west to Tunisia.

A U.S. military official said Monday Libyan government forces fired a scud missile for the first time since their conflict with the anti-government rebels began.

The official, who asked not to be named, said the missile landed in the desert about 80 kilometers outside Brega. Libyan forces and the rebels have battled over the strategic oil port city for months.

NATO and U.S. air strikes have targeted Libyan scud missile facilities and air defense sites out of concern that Mr. Gadhafi would use the missiles to target areas beyond government control.

Also Monday, a senior Libyan Interior Ministry official flew to Egypt with nine family members in what appears to be another defection from Mr. Gadhafi's government.

Nassr al-Mabrouk Abdullah and his relatives arrived in Cairo on a private jet from the Tunisian resort island of Djerba. Abdullah is believed to be a deputy interior minister. He entered Egypt on a tourist visa and did not meet any representatives of Mr. Gadhafi's embassy in Cairo.

Western news agencies quote sources in Djerba as saying Mr. Gadhafi's aides met Libyan rebels at a local hotel on Sunday. U.N. Secretary-General Ban Ki-moon's Libya envoy Abdul Ilah al-Khatib arrived in Tunisia Monday to join the talks between the two sides.

In an audio message broadcast on Libyan state television Monday, Mr. Gadhafi urged his people to fight to “to liberate Libya” from rebels who began their uprising in February to end his 42-year rule. He called the rebels “traitors” and denounced NATO as a “colonizer” for staging airstrikes in support of the uprising.