Lebanon's Shiite movement Hezbollah said Sunday that at least five of its fighters were killed along with "dozens" of gunmen in clashes in the Syria-Lebanon border area.
A Hezbollah security official, speaking to AFP on condition of anonymity, confirmed the deaths of five fighters after clashes around the town of Nabi Sbat in eastern Lebanon.
Hezbollah sources in the area reported earlier that the clashes erupted after gunmen from Syria attacked Hezbollah posts in the area.
"Hezbollah posts in the mountains around Nabi Sbat, east of Baalbek, were attacked by armed groups who came from Qalamun" in Syria, the Hezbollah source in the region said.
"Hezbollah returned fire, inflicting heavy losses among the attackers."
Lebanon's official National News Agency also reported the clashes, saying they erupted after an attack launched by gunmen from Asaal al-Ward in Syria's Qalamun province.
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.
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Lebanon's border with Syria is not officially defined and much of it is porous and unpatrolled, with local residents, smugglers and others moving freely across it.
Hezbollah maintains several military posts along inaccessible parts of the border, 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.
Lebanese army forces were also seen deploying towards the area where the clashes were taking place, the correspondent added.
The clashes come 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.