Updated

The United Nations' special envoy for Libya said Tuesday that he was meeting with representatives of both sides of the conflict, days after rebels made a significant advance that brought them within 30 miles of Muammar al-Qaddafi's stronghold in the capital Tripoli.

Abdel-Elah al-Khatib, Jordan's former foreign minister, arrived in the Tunisian capital Tunis on Monday for the meetings with representatives of both Qaddafi and the rebels. He said there were no direct negotiations as he met the two sides separately in the neighboring country. A Tunisian security official said al-Khatib might also meet with a representative of Venezuelan President Hugo Chavez. That envoy has been on the Tunisian isle of Djerba for the past days.

Back in Libya, a rebel advance over the weekend into the strategic city of Zawiya on the Mediterranean coast, just 30 miles from Tripoli, put the opposition force in the strongest position since the 6-month-old civil war began to attack the capital. Residents were fleeing Tripoli in long lines of cars, fearing the fighting would soon reach them.

The Obama administration said Monday that the U.S. was encouraged by the rebel advances and hoped they had broken a monthslong stalemate with Qaddafi's forces.

In a sign of the regime's growing distress, U.S. defense officials said Libyan government forces tapped into their stores of Scud missiles this weekend, firing one for the first time in the half-year conflict with rebels. No one was hurt. The missile was fired toward a second front line in the east of the country around the town of Brega.

The missile launch was detected by U.S. forces shortly after midnight Sunday and the Scud landed in the desert about 50 miles outside Brega, said one U.S. official, who spoke on condition of anonymity to discuss military operations. It was launched about 50 miles east of Sirte, a city on the Mediterranean coast about 230 miles east of Tripoli. Sirte is Qaddafi's hometown and a bastion of support for him.

Noting that Scuds are not precision guided missiles, officials said they couldn't tell if Brega was the target.

On Tuesday, rebels and Qaddafi forces fought for control of Zawiya on a main road leading from Tunisia in the west to Tripoli. Rebels are trying to cut off two major supply routes into the capital from Tunisia in the west and another in the south. The routes are critical with NATO imposing a no-fly zone over Libya. Rebels said Monday they also cut oil pipelines from Zawiya to Tripoli. Oil-rich Libya's only functioning refineries are in Zawiya.

Medics at a field hospital on the outskirts of Zawiya said that 15 people were killed Tuesday in an artillery strike, including a woman and a child.

On the second front in the east, NATO planes could be heard overhead in Brega as rebels patrolled a ghost town. Furniture and clothing were strewn all over the residential compound, and many houses were broken into, their windows shattered and walls pocked with bullet holes.

Smoke was seen rising from the industrial town as fighting raged.

Rebel and regime forces have battled over the strategic port city of Brega throughout the conflict, and control has swung back and forth between the two sides.

In Tripoli, government spokesman Moussa Ibrahim confirmed that former interior minister Nassr al-Mabrouk Abdullah had defected from the Qaddafi regime and left to Egypt.

"He was under psychological and social pressure and he could not resist it, but the battle continues," said Ibrahim.