Rebels to the west and east of Tripoli battled Libyan dictator Muammar Gaddafi's forces Wednesday night for control of oil facilities vital to the civil war's outcome.

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

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

In Zawiyah, 50 km (30 miles) 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.

Fifteen of the rebels were killed on Tuesday and three on Wednesday, said spokesman Mohammad Zawawi.

Libyan state television showed video of Gaddafi supporters at the Brega terminal Wednesday chanting the leader's name.

After 41 years of supreme power, the 69-year-old Gaddafi seems isolated. Rebel forces are closing in on the capital from the west, south and east. 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.

Zawiyah controls the western highway linking Tripoli to Tunisia. Gaddafi forces were holding the refinery there and harassing rebels in the city with shelling and sniper fire.

"There are some snipers inside the refinery facility. We control the gates of the refinery. We will be launching an operation to try to take control of it shortly," a rebel fighter, Abdulkarim Kashaba, told Reuters.

MASS GRAVE

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.

The spokesman said rebel forces were now outside a place called Hisha about 100 km (60 miles) west of Misrata on the road to Tripoli. "They are now on the coastal road," 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.

Gaddafi's green flags were still flying from a refinery building and an electrical pylon in Zawiyah. The rest of the city now flies the red, black and green flag of the rebels.

Streets were largely deserted apart from clusters of fighters. Shops were shuttered. Medical workers said three people were killed and 35 wounded on Tuesday, mostly civilians.

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.

Calado said there had been a sharp increase in the past week in the number of foreign nationals asking to be evacuated. He estimated that more than 300,000 foreigners remain in Tripoli, including many from the Philippines and Sri Lanka, as well as Libya's neighbors Chad, Egypt and Tunisia.

"We have received 2,000 requests at this point. The potential caseload is huge. We're exploring the possibility of land, sea and air evacuations," he told Reuters.

The rebel National Transitional Council in Benghazi 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."

(Additional reporting by Tom Miles in Geneva, Robert Birsel in Benghazi, Missy Ryan in Tripoli, Hamid Ould Ahmed in Algiers; Joseph Nasr in Berlin; Phil Stewart in Washington; Writing by Michael Roddy; Editing by Nick Macfie)