Rebel fighters are holding much of the east and parts of the west.

Gaddafi has seen areas under his control shrink as rebels advance on Tripoli from the west, south and east [Al Jazeera]

Momentum appears to have now firmly swung in the opposition's favour after months of near-deadlock, with the rebels holding much of the east and parts of the west.

'Gaddafi's days are numbered,' said Jeffrey Feltman, US Assistant Secretary of State, during a visit to the de-facto rebel capital of Benghazi. 'The best case scenario is for Gaddafi to step down now ... that's the best protection for civilians.''

Mustafa Abdel Jalil, an opposition leader, echoed the statement.

'The end is very near' for Gaddafi, said the chairman of the rebel National Transitional Council (NTC) on Saturday.

'We have contacts with people from the inner circle of Gaddafi,' he said. 'All evidence [shows] that the end is very near, with God's grace.'

The territory remaining under Gaddafi's control has been shrinking dramatically in the past three weeks, with opposition fighters advancing on the capital, from the west, south and east. Major supply routes into the capital have also fallen under rebel control.

Challenges ahead

In an indication of challenges ahead, however, rebel spokesman Ahmed Bani said his troops retreated from parts of the eastern town of Brega, losing the industrial section to Gaddafi's forces.

The oil port has frequently changed hands throughout the conflict. At a news conference, Bani said rebels captured the city of Zlitan, 140km southeast of Tripoli, after more than two months of fighting.

Al Jazeera's Jacky Rowland reports from Benghazi on the cracks and defections from within Gaddafi's regime

'Zlitan is now completely liberated after a severe fight, and for the first time I can say we have control over it,' Bani told reporters on Saturday.

As rebels battled for towns on either side of Tripoli on Saturday, fighting spilled across the border into Tunisia, where Libyan infiltrators clashed with Tunisian troops.

Tunisian security sources said their forces intercepted Libyan men in vehicles with weapons and fought them through the night in the desert. They reported several casualties.

Gaddafi's forces west of Az Zawiyah and near the Tunisian border have been effectively encircled and cut off from their supply lines. Tunisia has increased its army presence in the border area.

Residents of the southern Tunisian desert town of Douz told the Reuters news agency that helicopters were seen overhead and troops had been summoned from nearby towns to subdue the infiltrators, who rode in vehicles without number plates.

The Tunisian security sources did not say whether the armed men were rebels or supporters of Gaddafi. Residents said they believed they were Gaddafi supporters.

Tunisian officials also said a Tunisian army helicopter had crashed because of mechanical problems in the border area, killing the pilot and co-pilot.

'Gaddafi is finished'

The siege of Tripoli and the prospect of a battle for the capital have added urgency to the question of Gaddafi's fate. The leader has repeatedly vowed never to leave the country. Rebels say they will not stop fighting until he is gone.

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Representatives of the two sides held talks early this week in a Tunisian resort, attended by a former French prime minister, but announced no breakthrough.

The severing of the road link between Tripoli and Tunisia makes further talks difficult.

A Tunisian official source said Libya's senior oil official, Omran Abukraa, had arrived in Tunisia after deciding not to return to Tripoli from a trip to Italy.

If confirmed, it would be the third apparent defection of a senior Gaddafi associate this week. A senior security official arrived in Rome on Monday, and rebels said on Friday that Gaddafi's estranged former deputy Abdessalam Jalloud had joined their side.

In Az Zawiyah's central square on Saturday, residents were burning and stamping on a green Gaddafi flag.

'Gaddafi is finished. Civilians are starting to come back to the cities. Libya is finally free,' said one, who gave his name as Abu Khaled.

In a nearby alley, residents had gathered to stare at the bodies of two Gaddafi soldiers lying in the street. Gunfire and explosions could be heard in the distance.

Rebels said the main Gaddafi force had retreated about 10km east to the town of Jaddayim and was shelling Az Zawiyah from there.

The sudden imposition of a siege around Tripoli has trapped its residents and cut them off from fuel and food.

The International Organisation for Migration said on Friday it would organise a rescue operation to evacuate thousands of foreign workers, probably by sea.

Source: Al Jazeera and agencies