Two militants of the powerful Shiite Hezbollah movement were among four people killed in a firefight Saturday with Sunni gunmen in eastern Lebanon, a security official told AFP.
The bloodshed is the worst sectarian violence to hit Baalbek, a Hezbollah bastion across the border with Syria and home to a complex of famed ancient Roman temples.
It was the second clash in the city since Wednesday involving Shiite militants and Sunni gunmen.
Lebanon is deeply divided over the 30-month war in Syria.
Hezbollah fighters have helped President Bashar al-Assad 's forces in their efforts to crush a Sunni-led armed revolt which most Sunni Muslims in Lebanon support.
It was not immediately clear what triggered the clashes on Wednesday and Saturday, but both happened at a checkpoint manned by Hezbollah.
On Saturday members of the Shiah family, who like other Sunnis back the anti-Assad uprising, opened fire at the checkpoint, and the militants shot back, the official said.
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"The incident let to clashes that killed two Hezbollah members and wounded five people," the official said, adding the wounded included members of the Sunni family and passers-by.
A Sunni woman and one passer-by later died of their wounds, he added.
Hezbollah militants took to the streets of Baalbek after the clashes, an AFP correspondent said, adding that five Sunni-owned shops were torched by unknown arsonists.
He said the three-hour firefight also spread close to the ancient Roman archeological site, a complex of temples on UNESCO's World Heritage List.
On Wednesday, Hezbollah militants traded fire with Shiah clan members at the same checkpoint. Two passers-by and a Hezbollah members were wounded.
Hezbollah has set up checkpoints in the Baalbek region after deadly bombings tore through its main southern Beirut stronghold in July and August.
Fifty people were wounded in the first attack and 27 killed in the second.
The army said in a statement it would dispatch troops to Baalbek and pledged to hunt down "armed men" in the city.
On Monday the army deployed in southern Beirut, taking over similar checkpoints in a deal struck with the Shiite group.