BAALBEK, Lebanon - Gunmen crossed from Syria into eastern Lebanon on Sunday and attacked posts of the Shiite movement Hezbollah, sparking fierce clashes, a Hezbollah source told AFP.
"Hezbollah posts in the mountains around Nabi Sbat, east of Baalbek, were attacked by armed groups who came from Qalamun" in Syria, the source said.
"Hezbollah returned fire, inflicting heavy losses among the attackers." Residents from Nabi Sbat who fled the fighting told AFP they saw Hezbollah convoys picking up several wounded fighters from the group during the clashes.
Hezbollah maintains several military posts along inaccessible parts of the border with Syria, and it rarely gives official details on clashes with jihadists or other fighters.
The fighting could be heard in the city of Baalbek, several kilometres (miles) away, an AFP correspondent said.
The attack comes two months after jihadists from the Islamic State group and Al-Qaeda's Syrian affiliate Al-Nusra Front attacked Lebanese security forces in Arsal, which also lies on the Syrian border in eastern Lebanon.
The jihadists withdrew into the mountains around Arsal after a ceasefire, but took with them around 30 soldiers and policemen as hostages.
Three of them have since been executed, contributing to rising anxiety in Lebanon over the encroachment of jihadists and spillover from the war in neighbouring Syria.
Hezbollah has dispatched fighters to bolster President Bashar al-Assad's troops against an uprising that many of Lebanon's Sunnis support.
The conflict has exacerbated existing tensions in Lebanon, and made Hezbollah and strongholds of its support a target for extremists who have detonated bombs in several areas of Lebanon.