Libyan rebels launched an assault on Zawiyah's oil refinery on Wednesday to drive the last of the forces loyal to Muammar Gaddafi out of the city west of Tripoli and tighten their noose around the capital.

After 41 years of supreme power, 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 actual whereabouts are not known.

In Zawiyah, which controls the western highway linking Tripoli to Tunisia, Gaddafi forces were holding the refinery 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," said rebel fighter Abdulkarim Kashaba.

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

A rebel spokesman from the rebel-held city of Misrata to the east of Tripoli said rebels had found the buried bodies of civilians 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 people of Tripoli. A rebel commander said the pipeline linking it with Tripoli was severed on Tuesday.

Gaddafi's green flags were still flying from a refinery building and an electrical pylon. 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 and shops were shuttered. Medical workers said three people were killed and 35 injured on Tuesday, mostly civilians.

Libya's rebel National Transitional Council (NTC) have denied 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, however, sticks to its insistence Gaddafi must step down and leave Libya, saying talks ignoring this basic demand would be "unthinkable".

In Washington, U.S. Defense Secretary Leon Panetta on Tuesday said "the sense is that Gaddafi's days are numbered".

Libyan government spokesman Moussa Ibrahim told Libyan reporters: "They have been saying this for six months and we are still here."