Rebel fighters have encircled Brega, a key oil export terminal with a refinery and chemical plant which for months marked the eastern limit of Gaddafi's control, but are staying out of the centre, rebel spokeman Shamsiddin Abdulmolah said on Monday.

"The main body (of Gaddafi's forces) retreated to Ras Lanuf," Abdulmolah said by telephone. "I am told they have some four-wheel-drive trucks with machineguns spread out between Ras Lanuf and Bishr."

Ras Lanuf, another major oil export centre, lies some 100 km (62 miles) west of Brega.

He said rebels trying to take control of Brega had been hindered by landmines laid by Gaddafi loyalists across the town, on its desert outskirts and at its oil installations.

"The main (oil) installations are still mined and the remnants of Gaddafi's forces are still there," said Abdulmolah. "Cleaning them out is going to be a bit difficult without damaging the facilities themselves."

The rebels say the mines have taken a heavier toll than guns and rockets as they tried to dislodge Gaddafi's forces from Brega with the help of NATO aerial attacks.

Abdulmolah said 10 rebels were killed and 175 wounded on Saturday, and two were killed and 120 wounded on Sunday.

"The bulk of our forces are now past Brega and are heading towards Bishr and Ugayla," Abdulmolah said. "I am sure they will clash today or tomorrow in and around Bishr and Ugayla."

The rebellion blew up in the eastern city of Benghazi five months ago, but Gaddafi has held fast in the capital Tripoli despite heavy aerial attacks by the rebels' Western allies, causing strains among the foreign coalition partners.