Six people have been killed in clashes in the northern Lebanese city of Tripoli, in violence stoked by civil war in neighbouring Syria.

The clashes on Monday night in the northen city ended a week of relative calm after 29 people were killed last month in the deadliest fighting yet between supporters of Syrian President Bashar al-Assad and pro-rebel forces.

Three of the dead came from the Sunni Muslim Bab Tebbaneh neighbourhood. A total of 38 people were wounded, mainly by sniper fire in the city centre, sources in Tripoli said.

Meanwhile in Sidon, gunmen fired at Sunni leader Maher Hammoud as he headed towards his mosque for dawn prayers. They missed their target and fled when Hammoud's two guards returned fire.

Hammoud is seen as close to Hezbollah and has criticised a prominent Sidon Islamist, Sheikh Ahmed al-Assir, who has called on Lebanese Sunnis to head for Syria to fight Assad.

Elsewhere, security services reported another attack on a Sunni imam with connections to Hezbollah.

They said the car of Sheikh Ibrahim Mustafa Breidi came under machinegun attack in Lebanon's eastern Bekaa area and caught fire.

Lebanese factions support opposing sides in the conflict in Syria and have often clashed inside Lebanon.

Saad Hariri, the country's former prime minister, said what was happening in Tripoli was "unacceptable and should not continue under any pretext".

"The Lebanese army must assume its responsibility in safeguarding the city and not leave it to those who arouse discord," he said in a statement issued on Monday evening.