UPDATE 1: Rival factions clash for a second day, violence has now claimed lives of 8 people.
Fierce sectarian battles raged in north Lebanon for a second day on Monday as the army vowed to use force if needed to contain the violence threatening to derail an accord to end the country's political crisis.
Two people were killed and two others died of wounds sustained a day earlier, bringing to eight the death toll from the clashes that erupted on Sunday in the port city of Tripoli, a security official told newswire AFP.
He said 45 other people were wounded.
The clashes in the densely populated Bab Al-Tebbaneh and Jabal Mohsen districts pitted Sunni supporters of the Western-backed majority against members of an Alawite sect loyal to the Hezbollah-led opposition.
Fighters traded heavy machine-gun fire, mortars and rocket-propelled grenades, prompting residents to flee the area or to hide in underground shelters.
Several homes as well as a gas station were burned.
A Lebanese army spokesman said troops planned to resort to force if necessary to stop the fighting.
Back-up troops could be seen deploying in the area where the battles were taking place by late afternoon.
"We will use force if needed to end all armed presence in the combat zones where we are sending reinforcements following a unanimous accord between the warring sides calling for a ceasefire," the spokesman said.
Representatives from the Sunni militant side and the Alawites, a secretive offshoot of Shi'ite Islam to which Syrian President Bashar Al-Assad belongs, had agreed to a ceasefire on Sunday but the accord was not respected.
The clashes in Tripoli have raised fears of a nationwide deterioration of the security situation amid stalled efforts by Prime Minister Fuad Siniora to form a national unity government following a deal last month that ended an 18-month political crisis.
Violence has erupted in various parts of the country in recent days between supporters of the ruling majority and militants from the opposition backed by Syria and Iran.
The latest flare-up came as Arab and European leaders met in Vienna on Monday to raise funds for a Palestinian refugee camp near Tripoli that was destroyed in clashes last summer between Islamists and Lebanese troops.
The security situation in Lebanon's Palestinian refugee camps is also of concern following several incidents, notably in the largest camp of Ain El-Helweh, considered a hotbed for Islamist extremists.
Imad Yassin, a senior member of the Jund Al-Sham Islamist group, was seriously wounded on Sunday in a blast near the camp in southern Lebanon along with two of his bodyguards.
Sectarian clashes in various parts of the country in May left at least 65 people dead and stoked fears that Lebanon, which endured 15 years of civil war up to 1990, was heading for a new conflict.
An accord reached in the Qatari capital Doha on May 21 between the opposition and ruling coalition resulted in the election of Michel Sleiman as president, ending a six-month vacuum in the top job.
But the initial euphoria that greeted Sleiman's election has been replaced by a growing sense of dread as rival factions continue to bicker over the formation of the new government.
The Doha accord calls for the opposition to have veto power over key decisions in the unity cabinet and the drafting of a new electoral law ahead of legislative elections due next year.