At least 18 people, including 16 Sunni fighters, have been killed in clashes after they reportedly attacked posts of the Shia group Hezbollah in eastern Lebanon.
Reports said that fighters linked to al-Qaeda's Syrian wing, the Nusra Front, crossed into Lebanon on Sunday and attacked at least 10 Hezbollah-controlled bases along a mountainous range close to the Syrian border.
Sources told the Reuters news agency that the clashes killed at least 16 Sunni fighters, adding that Hezbollah fighters from other parts of Lebanon had gone to defend the area.
At least two Hezbollah fighters were also reported killed.
Hezbollah's al-Manar television reported late on Sunday that the clashes had ended and the group had pushed the Sunni fighters back over the border.
Armed with mortars and rocket-propelled grenades, the Sunni fighters attacked a large area of territory stretching from south of the town of Baalbek up to areas close to the border town of Arsal.
Hezbollah maintains several military posts along inaccessible parts of Lebanon's border with Syria, and rarely gives official details on clashes with opposition fighters.
Violence from Syria has often spread into Arsal and surrounding areas, where the Lebanese army has also battled fighters from Syria
In August, ISIL and Nusra fighters stormed Arsal in the worst spillover of Syria's war into its neighbour to date, killing and capturing members of the Lebanese military.
They have since killed at least three of the captive soldiers.
The conflict has exacerbated existing tensions in Lebanon, and made Hezbollah and strongholds of its support a target.
Hezbollah has dispatched fighters to bolster President Bashar al-Assad's troops against an uprising that many of Lebanon's Sunnis support.