Muammar Gaddafi's regime appears to be crumbling from within after the third defection of a senior member of his regime within days.

As rebels continued to advance on the battlefield, it was reported that Abdel-Salam Jalloud, who helped propel Gaddafi to power in 1969 and was for decades his powerful deputy, flew out of Djerba airport in Tunisia early on Saturday . Rebels attempting to oust Gaddafi claimed he had defected to their side, though this could not be independently confirmed.

Jalloud's apparent departure follows the reported defectionearlier this of oil minister Omrane Boukraa and a senior security official, Nasser al-Mabruk Abdullah, who fled to Cairo from Tunisia on Monday with his family.

A swirl of rumours now circulates concerning the intentions of Gaddafi and his family, as rebel forces continue operations on three fronts to cut off the capital, Tripoli.

In Zlitan, a town formerly largely loyal to Gaddafi that was captured on Friday, rebels continued with street-to-street searches, while rebel forces also claimed the final capture of Brega, which has changed hands on a number of occasions.

In Zawiyah, 30 miles from Tripoli, that earlier this year endured a long and bloody siege, the city centre is under rebel control, but was heavily shelled by government artilleryon Saturday .

The territory remaining under Gaddafi's control has been shrinking dramatically in the past three weeks, with opposition fighters moving closer to Tripoli, a metropolis of two million people, from the west, south and east.

Nato's bombing campaign has made it difficult for the regime to send massive reinforcements to Zawiyah, the rebels' biggest prize in months.

But Friday's onslaught by regime forces signalled that an opposition push toward Tripoli could be arduous and bloody. At one point some two dozen rebel fighters were pinned down behind a building about 200 yards from Zawiyah's central square.

On the western front, Abdullah Maiteeg, 24, nicknamed Grandee, described the assault on the town of Zlitan, where more than 30 rebels lost their lives in heavy fighting. He was in the town late on Friday when shattering explosions erupted around him, ripping through nearby fighters.

"There was a tank in front of us, a T-72 I think," he said. "A lot of guys were killed by this tank."

Crouching amid sand and rocks, Maiteeg saw a bright flash followed by a huge explosion as the tank was hit by a bomb from a jet circling high above: "Nato came and destroyed the tank, and that was it."

Leading the assault, men of Maiteeg's Alla Khasheem brigade surged forward into the town, which they secured after several hours of vicious street fighting that left 35 rebels dead and more than 150 wounded.

Maiteeg added he thought the air strike had been called in by a British forward air controller who has become a familiar sight on the front lines west of Misrata in recent weeks. "There was a British guy on the left side [of the highway]," he added.

Unwilling to risk casualties to the civilian population, opposition commanders delayed the attack until, late on Thursday night, with Gaddafi's front lines buckling across Libya, town leaders finally agreed to change sides.

"The problem [in Zlitan] is we don't know who is with us and who is against us," Maiteeg said. "We know a few rebels there, they are good guys, but the rest we don't know. We don't want to get shot in the back."

As rebel forces prepare for a final push on Tripoli, commanders say that they are having to consider whether the populations will welcome them. It took three months to secure "permission" to enter Zlitan.

In a further development, Tunisia's army clashed overnight with an armed group of Libyans who had infiltrated into Tunisia, a Tunisian military source said.

Earlier, a security source said that overnight fighting between Tunisian forces and the armed men had caused several casualties. The Libyans were intercepted in vehicles with weapons in the desert in the south-east of Tunisia. No explanation has been supplied for the clash.