TRIPOLI, Lebanon — Military aid began arriving today after the United States said it will rush supplies to the Lebanese army fighting al-Qaida-inspired Islamic militants barricaded inside a Palestinian refugee camp in the country's north.

Meanwhile, sporadic gunfire exchanges early this morning 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 Nahr el-Bared. 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 U.S. officials said the military aid to Lebanon had been agreed to before the fighting broke out this week, the speedy shipment today marked the first tangible U.S. backing of the Lebanese authorities in their fight with the militants.

Secretary of State Condoleezza Rice on Thursday renewed the Bush administration's strong support of 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 destabilize democratic governments."

U.S. military assistance was renewed after Syria withdrew troops from Lebanon in 2005, and increased after last year's summer war between Lebanese Hezbollah militants and Israel.

Hoping that a boosted army could eventually disarm Hezbollah, the U.S. 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 armor, anti-aircraft missiles and the absence of an air force.

At the camp, gunfire had grown heavier Thursday shortly after nightfall and continued on and off through early today, as both sides probe each other's defenses. Troops have entrenched their positions around the camp but did not appear to be attempting to advance. Reinforcements from other regions are also arriving, mostly drawn from elite commando units.

Two military transports landed at the Beirut airport this morning, spotted by many residents of the capital. The military refused to comment, apparently because of the sensitivity of the matter.

Security officials, speaking on condition of anonymity, said a United Arab Emirates air force plane landed late Thursday with the first supplies.