Lebanese troops staged a broad daylight assault on a building in Tripoli where militants from Fatah al-Islam were holed up after a morning of deadly shootouts in the northern port city and a nearby Palestinian refugee camp.

The army said 13 soldiers lost their lives in the deadliest fighting with Islamists for seven years, while eight gunmen were killed along with a civilian who was caught in the crossfire when troops attacked the building in Tripoli.

Lebanon sent in troop reinforcements to contain the battles involving anti-tank rockets and canons which erupted at dawn in Tripoli and around the nearby Palestinian refugee camp of Nahr al-Bared, a Fatah al-Islam stronghold.

The sound of gunfire rattled through the streets of Tripoli throughout the morning, where soldiers in jeeps and at least one tank were seen rolling through the streets and plumes of smoke billowed into the sky.

"The blows dealt by Fatah al-Islam against the Lebanese army are a premeditated crime and a dangerous attempt to destabilise (Lebanon)," charged Prime Minister Fuad Siniora, whose Western-backed government has been paralysed for months by an acute political crisis.

The army said three militants were killed in the assault on the building in the residential district of Miteyn, along with a civilian and an army captain, while dozens had been arrested, but that gunmen were still hiding out in other parts of Tripoli.

"We warn the army against continuing its provocations against our mujahedeen (Islamic fighters) or risk us opening fire against it and all of Lebanon," according to a statement said to have been issued by Fatah al-Islam.

A convoy of about 100 troop carriers, jeeps and ambulances estimated to contain between 800 and 1,000 men was seen moving north on the road between Beirut and Tripoli, Lebanon's second largest city.

By longstanding convention, the Lebanese police and army do not enter Lebanon's 12 refugee camps, leaving security there to Palestinian militant groups.

The army said the fighting was triggered when the militants staged an attack on a military post outside Nahr al-Bared, home to about 22,000 refugees.

In the deadliest attack, seven soldiers were killed when their patrol was ambushed in Qalamun south of Tripoli.

Syria, the former power broker in Lebanon, announced it had closed two border posts into its smaller neighbour because of the violence.

Russia expressed deep concern, saying: "Such an upsurge of violence in an already tense situation in Lebanon gives rise to deep anxiety."