One person was killed and seven others were wounded in a clash in the heart of Tripoli in northern Lebanon on Thursday, a security official told AFP, in the latest in a string of Syria-related incidents.

The army deployed in the souk area, restoring a tense calm four hours after the clash broke out between Salafists who support the revolt in Syria and pro-Damascus fighters, the source said.

Two of the wounded were soldiers, though the identity of the man killed in the firefight in the markets of central Tripoli had not been confirmed, the official said on condition of anonymity.

It was the first battle since 2008 in central Tripoli, although frequent Syria-related violence has raged in other districts.

It comes after some three weeks of sectarian fighting in the flashpoint Jabal Mohsen and Bab al-Tebbaneh neighbourhoods, during which around 40 people were killed.

An AFP journalist saw artillery and military vehicles deployed at the entrance to Tripoli, which has seen escalating violence since the March 2011 outbreak of a revolt in Syria against President Bashar al-Assad.

Thursday's battle saw Salafists pitted against members of the Syrian National Socialist Party, which supports the Assad regime.

It came a day after two gunmen, including a Syrian, were killed in a firefight with Lebanese soldiers near Arsal on the border with Syria, the army said.

On Wednesday night, "a group of armed men driving a pick-up truck assaulted and opened fire on a Lebanese army checkpoint in the Wadi Hmeid area near Arsal," the army said, adding that two attackers were killed.

Hours earlier a Syrian army helicopter bombarded areas near Arsal, which is home to a majority Sunni population that supports the revolt against Assad.

The Lebanese army is working to stop the smuggling of weapons and fighters across the border into Syria.

On Wednesday, the strategic Syrian town of Qusayr near the border fell to troops loyal to Assad, including fighters from Lebanon's Hezbollah movement.

Hours after Qusayr's capture, 10 rockets fired from inside Syria hit the Hezbollah bastion of Baalbek in eastern Lebanon, the army said.

President Michel Sleiman on Thursday condemned the violence and called for "a firm and strong response to the sources of fire".

Although Lebanon has officially adopted a position of neutrality in Syria's raging conflict, it is sharply divided.

The Shiite movement Hezbollah backs Assad's regime, with fighters on the ground, while the Sunni-led March 14 bloc supports the revolt.