Libyan rebels on Saturday said they had launched a push to capture the coastal oil town of Brega, but were advancing slowly because Muammar Gaddafi's forces had sown landmines on the approaches to the town.

"There's a big movement on all fronts around Brega, we are attacking from three sides," said rebel spokesman Mohammad Zawawi.

Fighting on the eastern front of the civil war, which has ebbed backwards and forwards for the past months, has been bogged down for weeks on the fringes of Brega, southwest of the rebel capital Benghazi.

Zawawi said rebel forces were in sight of a residential area of Brega and believed they could take the town.

"It could be very soon, but we don't want to lose anybody so we're moving slowly but surely," he said.

Friday was another day of claim and counter-claim in the six-month-old war.

Gaddafi's government denied a rebel report that a Nato air strike had killed the Libyan leader's son Khamis, commander of one of the government's most loyal and best-equipped units.

Government spokesman Moussa Ibrahim said word of Khamis Gaddafi's death was a ploy to cover up the killing of three civilians in Zlitan, a battlefront city where Gaddafi forces are trying to halt the rebel advance on Tripoli.

"It's false news. This is a dirty trick to cover up their crime in Zlitan," he told Reuters.

A rebel spokesman said the Nato air strike had killed 32 Gaddafi loyalists in Zlitan, where Khamis Gaddafi's elite 32nd Brigade is believed to have been leading the defence of the approaches to Tripoli, 160 km (100 miles) away.

Nato said it had targeted a command-and-control target in the Zlitan area but "cannot confirm anything right now because we don't have people on the ground".

Rebels who cleared Gaddafi's forces from Libya's third-largest city Misrata after weeks of intense fighting have been trying to push westwards and take Zlitan, which would open the coastal road toward his Tripoli stronghold.

Khamis has been "killed" twice in the past six months: Arab media reported in March that he had died in a kamikaze crash by a disaffected Libyan air force pilot. Libyan state television countered with pictures of a man resembling Khamis, which it said disproved the report of his death. 

The government said earlier this year that a Nato strike in Tripoli had killed Gaddafi's son Saif al-Arab, who unlike Khamis did not have a high public profile or a major leadership role.

Gaddafi has kept control of the capital despite severe fuel shortages and rebel advances backed since March by Western air strikes, assault helicopter attacks and naval bombardments.

The rebels face numerous problems, from stalling battlefield momentum to internal splits, exposed starkly last week when military chief Abdel Fattah Younes was assassinated behind his own lines in circumstances yet to be explained.

Near the capital, rebels also control the Western Mountains southwest of Tripoli. A rebel official there, Colonel Juma Ibrahim, told Reuters his forces had set an ultimatum to the surrounded town of Tiji to surrender or face attack on Saturday.

Rebels were using loud-hailers to appeal to a tribal chief close to Gaddafi to evacuate civilians from Tiji and broker the withdrawal of pro-Gaddafi forces.

"If he does not comply, we will attack," said Ibrahim. – Reuters