BEIRUT (AP) -- Lebanese supporters and opponents of Syrian President Bashar Assad fired heavy machine guns and lobbed mortar shells at each other Thursday in some of the worst fighting in the port city of Tripoli in years.
The battles raised the five-day death toll to 16 and fed fears of the Syrian civil war spreading to Lebanon and other neighboring countries.
The violence also added to the urgency to U.S.-Russian efforts to bring both sides of the Syrian conflict to a peace conference in Geneva. Members of the Syrian opposition began three day meetings in Istanbul to hash out a unified position on whether to attend, while maintaining that Assad's departure from power should be the goals of the negotiations.
Lebanon has been on edge since the uprising in Syria began in March 2011. The country, which is still struggling to recover from its own 15-year civil war, is sharply divided along sectarian lines and into pro and anti-Assad camps. The overt involvement by the Iranian-backed Hezbollah Shiite militant group alongside Assad's regime has sparked outrage among many Sunnis in Lebanon who identify with the overwhelmingly Sunni rebels fighting to topple Assad.
Deadly sectarian street fighting has erupted on several occasions, mostly in Tripoli, Lebanon's largest city and a hotbed for Sunni Islamists. This week's fighting there has been linked to a Syrian regime offensive against the rebel-held city of Qusair in western Syria that has included Hezbollah fighters supporting Syrian troops against the rebels.
Tripoli is overwhelmingly Sunni but has a tiny community of Alawites, members of Assad's minority sect, which is an offshoot of Shiite Islam.
Residents reported more than six hours of fighting that began late Wednesday and continued through Thursday morning. Mortar shells were used for the first time. Ambulances rushed back and forth, transporting casualties to hospitals as officials used mosque loudspeakers to urge citizens to take shelter in basements. Schools and many businesses were shuttered Thursday as sporadic fighting continued.
Five people were killed, pushing the overall death toll to 16 since fighting began Sunday, with 200 people wounded, a security official said, speaking on condition of anonymity in line with military regulations.
"It was a frightful night that instilled terror in the heart of every resident of Tripoli," said Shada Dabliz, a 40-year-old peace activist in the city. "Tripoli is part of Lebanon, where is the state? Why doesn't the government do anything?"
Cabinet minister Faisal Karami said the fighting was among the worst in the city since Lebanon's civil war that ended in 1990, according to comments reported by Lebanon's state-run National News Agency.
Ashraf Rifi, a former police chief who has a large Sunni following in Tripoli, said the flare-up in Tripoli was a direct result of Hezbollah's involvement in Syria and accused the group of "trying to deflect attention" from its participation in the fighting in Syria.
Hezbollah and its allies held a dominant role in the Lebanese government, which resigned in March but continues to function on a caretaker basis. Various Lebanese factions have been unable to agree on the formation of a new government.
Fighting in Qusair continued for a fifth day Thursday, after Syrian opposition leaders urged rebels from elsewhere to converge on the town, which is strategically important to both sides.
The regime would solidify control in the heavily populated west if it retakes the town, which links the capital Damascus with the Alawite heartland along the Mediterranean coast. For the rebels, predominantly Sunni Qusair is part of a supply line of weapons and fighters from Lebanon.
The Britain-based Syrian Observatory for Human Rights, a pro-opposition group, said Thursday that 46 Hezbollah fighters have been killed in the battle for Qusair. In the past, Hezbollah tried to play down its involvement in the civil war, but its high-profile role in Qusair has made that impossible. Hezbollah has held funerals for fighters who officials close to the group say died at Qusair.
Overall, at least 104 Hezbollah fighters have been killed in Syria in recent months, according to the Observatory, which relies on a network of sources in Syria.
Hezbollah's growing involvement has prompted international condemnation. European officials said Wednesday that the European Union is reassessing whether to declare Hezbollah a terrorist organization, a move it has long shied from despite pressure from the U.S.
U.S. Secretary of State John Kerry said Wednesday that Hezbollah's overt engagement across the border puts Lebanon at risk.
"You have this major force in Lebanon, Hezbollah ... which has chosen, on behalf of all of the Lebanese people, to drag them into this," he said at a press conference in Amman, Jordan. "That's exactly the kind of danger that we are trying to avoid."