ZAWIYAH  - Rebels to the west and east of Libya's capital fought forces loyal to Muammar Gaddafi for control of oil facilities vital to winning the six-month-old civil war.

The United States also deployed two more Predator drones for surveillance operations over Libya, a US official told Reuters on Wednesday, as Gaddafi's forces faced unprecedented pressure.

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The drones arrived earlier this week, the official said, speaking on condition of anonymity. It was not immediately clear how many US drones were currently deployed on the NATO mission.

In Zawiyah, 50 km west of Tripoli, rebels assaulted a coastal oil refinery to try to drive the last Gaddafi forces out and tighten their noose around the capital.

A rebel spokesman said a pipeline to Tripoli was cut. There was no word on the outcome of their assault after nightfall.

In Brega, on the eastern front, rebel forces said they had suffered 18 killed and 33 wounded on Tuesday and Wednesday in their battle to dislodge Gaddafi forces from the oil port and refinery, where they have been fighting for many days.

After 41 years of supreme power, the 69-year-old Gaddafi seems isolated. Rebel forces are closing in from the west, south and east, cutting off his Tripoli stronghold on the Mediterranean shore. Gaddafi's whereabouts are not known.

Aided by NATO's fighter-bombers, assault helicopters and a naval blockade, the rebels have transformed the battle in the last few days after many weeks of stalemate.

A rebel spokesman from the opposition-held city of Misrata to the east of Tripoli said rebels had found the buried bodies of civilians they said had been slaughtered by Gaddafi forces.

"We discovered a mass grave containing 150 bodies in Tawargha. These are the corpses of civilians kidnapped from Misrata by Gaddafi's loyalists," he said. Rebels found a video "showing kidnappers cutting the throats of people", he said.

Zawiyah's refinery is one of the few sources of fuel for Gaddafi's troops and the people of Tripoli. A rebel commander said the pipeline linking it to Tripoli was severed on Tuesday.

If the pipeline to Tripoli is indeed cut, "that would imply dire consequences for the population in Tripoli in terms of fuel supplies needed for the city to keep operating", said Fernando Calado of the International Organization for Migration.

Libya's rebel National Transitional Council (NTC) denies holding secret talks with Gaddafi to end the war. But suspicions persist that some form of end-game negotiation may be going on.

The NTC insists Gaddafi should step down and leave Libya, saying talks ignoring this basic demand would be "unthinkable".

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