Sporadic gunfire exchanges early Friday punctured the lull in the fighting as the Lebanese army continued to build up around the Nahr el-Bared refugee camp near the port city of Tripoli.

The move appeared to be either a preparation to storm the camp a maze of narrow streets and tightly packed residential buildings where hundreds of Fatah Islam militants are holed up or a tightening of the siege to force them to surrender. Thousands of Palestinian refugees are also trapped inside.

A deputy Fatah Islam leader threatened more violence if the army attacks. Abu Hureira told the pan-Arab Al Hayat daily by telephone that "sleeper cells" in other Palestinian camps and elsewhere in Lebanon were awaiting word for a "violent response."

Although US officials said the military aid to Lebanon had been agreed to before the fighting broke out this week, the speedy shipment Friday marked the first tangible US backing of the Lebanese authorities' fight with the militants.

By early afternoon Friday, a total of five military transport planes landed at the Beirut airport, including one from the US Air Force, two from the Emirates' air force and two Royal Jordanian Air Force planes. Both Jordan and Emirates are close US allies.

Secretary of State Condoleezza Rice on Thursday renewed the Bush administration's support for the Lebanese government.

"I certainly hope that the Lebanese government will be able to deal with these extremists," Rice said. "It's just another example of extremists in the Middle East who are trying to destabilise democratic governments."

US military assistance, limited during Syria's control of Lebanon until 2005, increased after last year's summer war between Lebanese Hezbollah militants and Israel.

Hoping that a boosted army could eventually disarm Hezbollah, the US has pledged $40 million in military aid. Lebanon's 70,000-strong army is underarmed and overstretched, with army leaders complaining of a lack of heavy armour, anti-aircraft missiles and the absence of an air force.

At the camp, Lebanese troops entrenched their positions around the camp but did not appear to be attempting to advance. Reinforcements from other regions were also arriving, mostly from elite commando units.