A sniper killed a Sunni sheikh in the north Lebanon city of Tripoli on Friday, sparking new clashes between pro- and anti-Syrian factions that dashed a tenuous truce, a security official told AFP.

The death of Sheikh Khaled al-Baradei, 28, brought to 12 the number of people killed in fighting in the city over the past five days and stoked fears of a spillover of major violence from the conflict inneighbouring Syria.

A further 86 people have been wounded.

The exchanges of rocket-propelled grenade and small arms fire pitted fighters from the anti-Syrian Sunni Muslim Qobbeh district against those from the neighbouring pro-Damascus Alawite district of Jabal Mohsen, an AFP correspondent reported.

The intensity of the exchanges sparked large fires in the two neighbourhoods in the east of the Mediterranean port city,Lebanon's second largest.

Families hammered holes through the walls of their apartments to escape to safety down makeshift ladders as the clashes raged.

The fighting continued until around 8:30 am (0530 GMT) when militiamen on both sides pulled back from the frontline and a fragile calm returned.

"We were suprised by this battle," said Abu Othman, a gunman from the Sunni side. "They are the ones who opened fire, the Jabal Mohsen people."

Hundreds of soldiers with tanks and military vehicles have deployed on the aptly named Syria Street -- which acts both as the dividing line between the two districts and as the frontline when fighting erupts.

Several families displaced by the fighting had returned to the two districts on Thursday to inspect the damage to their homes, as a truce agreed on Wednesday had appeared to take hold.

"I can no longer cope with this situation. In my house I have got three families who have fled the violence," said Ahmed Breiss, who runs a car workshop in Qobbeh.

"We have nothing to do with what's going on in Syria. We want to live in peace," he said.

"We've got barely enough to survive but the militiamen get wages. They're not fighting for any cause just for their own interests."

A wave of kidnappings preceded the latest round of fighting and rattled the already fragile security situation in Lebanon, which lived under three decades of Syrian domination.

Prime Minister Najib Mikati, a native of Tripoli, on Wednesday raised fresh concern over "efforts to drag Lebanon more and more into the conflict in Syria when what is required is for leaders to cooperate... to protect Lebanon from the danger."

The authorities have instructed the army and security forces "to bring the situation under control, to prohibit any armed presence and to arrest those implicated" in the violence, he said in a statement.

The United Nations has called for more international support for Lebanese authorities to prevent a spillover of the 17-month conflict in neighbouring Syria.

"The situation in Lebanon has become more precarious and the need for continued international support to the government and the Lebanese armed forces increasingly important," UN under secretary general Jeffrey Feltman said on Wednesday.