TUNIS/ZAWIYAH, Libya (Reuters) - Rebels battled for towns on either side of the Libyan capital Tripoli on Saturday, and fighting spilled across the border into Tunisia where Libyan infiltrators clashed with Tunisian troops.
The United States said Muammar Gaddafi's "days are numbered" as insurgents, backed by NATO air strikes, put his four-decade rule in the North African nation under unprecedented pressure amid reports of more defections from his ranks.
Tunisian security sources said their forces had intercepted Libyan men in vehicles with weapons and fought them through the night in the desert. They reported several casualties.
The six-month-old war in Libya came close to the frontier this week after rebels suddenly seized the coastal city of Zawiyah just 50 km (30 miles) west of Tripoli, surrounding the capital and severing its supply routes.
Intense fighting continued in Zawiyah on Saturday and rebels occupying the center of the city said pro-Gaddafi forces showed no sign of retreating to the capital.
Gaddafi soldiers west of Zawiyah and near the Tunisian border have been effectively encircled and cut off from their supply lines, and Tunisia has beefed up its military presence in the border area.
Residents of the southern Tunisian desert town of Douz told Reuters by telephone that helicopters were swooping 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, but 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.
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.
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.
A senior U.S. official said on Saturday that Gaddafi's days are numbered and that the opposition must prepare for power.
"It is clear that the situation is moving against Gaddafi," U.S. assistant Secretary of State Jeffrey Feltman told a news conference after meeting Libyan rebel leaders at their headquarters in Libya's eastern city of Benghazi.
"The opposition continues to make substantial gains on the ground while his forces grow weaker," Feltman said.
The United States is among the more than 30 nations that have recognised the rebels' National Transitional Council (NTC) as Libya's legitimate authority.
A Tunisian official source said Libya's top 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 Abdel Salam Jalloud had joined their side in the western mountains.
Mortar and rocket rounds crashed into the centre of Zawiyah on Saturday. Shells struck the central hospital around dawn, blasting holes in the walls.
"Gaddafi will try to take back Zawiyah at any price. He will keep shelling the hospital," said a rebel fighter as he was preparing for midday prayers in the mosque of Bir Hawisa, a nearby village where many civilians are sheltering.
"We will not let that happen. We will fight," he said.
Rebels said the main Gaddafi force had withdrawn to a village 10 km to the east. On Saturday the area around Zawiyah's main hospital showed the signs of battle, with buildings punctured by artillery blasts and licked by flames.
In the central square, 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.
East of the capital, where fighting has been bloodier and rebel advances far slower, opposition forces fought street battles in the city of Zlitan but suffered heavy casualties, a Reuters reporter said on Friday. A rebel spokesman said 32 rebel fighters were killed and 150 wounded.
Gaddafi's spokesman Moussa Ibrahim said late on Friday the government's military held the upper hand in both cities.
The sudden imposition of a siege around Tripoli has trapped its residents behind the front line and cut it 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.
Additional reporting by Missy Ryan in Tripoli and Robert Birsel in Benghazi, Libya; Writing by Richard Valdmanis and Peter Graff; Editing by Maria Golovnina