"Three of our soldiers were killed and a dozen wounded by Fatah al-Islam snipers," an army spokesman said, bringing the death toll from three weeks of fighting between Islamists and the army to 112, 51 of them soldiers.

The unrest, which has also seen at least eight bomb or grenade attacks in and around the capital, is by far Lebanon's deadliest internal strife since the 1975-90 civil war.

The resulting insecurity has prompted many schools to start their summer holidays early and has further dented an economy still reeling from last year's devastating war with Israel.

A military commander outside the besieged Nahr al-Bared refugee camp said four of the wounded soldiers were in a critical condition.

He said that the soldiers had met fierce resistance when they attempted to capture militant positions on the northeastern outskirts of the camp.

"Our soldiers are fighting from high-rise to high-rise but are encountering fierce resistance from the extremists who have boody-trapped the buildings," the officer said, asking not to be identified.

"The army is advancing slowly but surely," he added.

An AFP correspondent heard heavy shelling of the tower blocks where the fighters of fringe militant group Fatah al-Islam are dug in.

Fatah al-Islam spokesman Shahine Shahine said the bombardment had been cover for a ground assault on militant positions on the camp's outskirts. "The assault was repulsed," he added.

The fighting came as a group of Muslim clerics that has been shuttling between the two sides in a bid to broker a peaceful end to the siege was due to meet army chief Michel Suleiman.

The mediators said they had suffered a setback on Friday when they were only able to see Shahine not more senior Fatah al-Islam leaders.