The United Nations' special envoy for Libya is meeting representatives of both sides of the conflict.

It comes days after rebels made a significant advance that brought them within 30 miles of Muammar Gaddafi's stronghold in the capital Tripoli.

Abdel-Elah al-Khatib, Jordan's former foreign minister, arrived in the Tunisian capital Tunis yesterday for the meetings with representatives of Gaddafi and the rebels.

He said there were no direct negotiations as he met the two sides separately in the neighbouring country.

A Tunisian security official said al-Khatib might also meet a representative of Venezuelan president Hugo Chavez. The representative has been on the Tunisian isle of Djerba for a few days.

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 to attack the capital since the 6-month civil war began.

Residents were fleeing Tripoli in long lines of cars, fearing the fighting would soon reach them.

President Barack Obama's administration said yesterday the US was encouraged by the rebel advances and hoped they had broken a long stalemate with Gaddafi's forces.

In a sign of the regime's growing distress, US defence officials said Libyan government forces tapped into their stores of Scud missiles this weekend, firing one for the first time.

The missile was fired toward a second front line in the east of the country around the town of Brega and nobody was hurt.

The missile launch was detected by US forces shortly after midnight on Sunday and the Scud landed in the desert about 50 miles outside Brega, said one US official. It was launched about 50 miles east of Gaddafi's hometown Sirte, a city on the Mediterranean coast.

Today, rebels and Gaddafi forces fought for control of Zawiya on a main road leading from Tunisia to Tripoli.

Rebels are trying to cut off two major supply routes into the capital from Tunisia and another in the south. The routes are critical as Nato has imposed a no-fly zone over Libya. Rebels said yesterday 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 15 people were killed today in an artillery strike, including a woman and child.

On the second front in the east, Nato planes could be heard over 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 Gaddafi regime and left for Egypt.

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

A Tunisian security official said the discussions centred on a "peaceful transition" in Libya. The official said the rebels reacted angrily to the proposal with one member of their delegation throwing a shoe during the meeting to show his deep disdain.

Al-Khatib did not identify who he met or say what they discussed when he spke to reporters after a meeting today with Tunisian Foreign Minister Mouldi Kefi al-Khatib.

The UN denied its special envoy was taking part in the meetings. In a statement, it said it had "no concrete information about talks supposedly taking place in Tunisia".

Meanwhile, Moscow has said it was "deeply disturbed" that Nato had "overstepped" its aerial campaign in Libya.

A Russian official said his country, as well as some other members of the UN Security Council, were unhappy with the destruction of infrastructure and attacks on power supplies in government-controlled areas.

A Nato spokesman denied that the alliance was overstepping its mandate.

"We take the side of the people of Libya," he said. "When we strike a tank, it is because we understand it does represent a threat to the local population."