BEIRUT (Reuters) - Ten fighters from Lebanon's Shi'ite Hezbollah group were killed in clashes with fighters from al-Qaeda's Syrian wing in eastern Lebanon on Sunday, a source close to the group said on Monday.
The death toll was one of the highest the group has suffered in a single action since it said it would fight alongside the forces of Syrian President Bashar al-Assad.
The move by Iran-backed Hezbollah, announced last year, has helped turn the tide of civil war in Syria against insurgents seeking to oust Assad.
Hezbollah pushed back a major offensive on Sunday by hundreds of fighters from the Nusra Front who attacked several bases of the group along a mountainous range close to the border, the latest spillover of violence from Syria's civil war.
Sources said dozens of Nusra fighters were also killed.
Sunday's two-hour battle, in which both sides used mortars and rocket-propelled grenades, is one of the deadliest fought in Lebanon between the Shi'ite group and Syrian insurgents.
The Qaeda fighters, who see Hezbollah as among their chief foes, attacked a large area stretching from south of the town of Baalbek up to areas close to the border town of Arsal.
The Lebanese army stayed out of the clashes but has also battled insurgents in the area.
Lebanon, which hosts around a million and a half Syrian refugees, has also seen violence spill over the border from Syria with bombings in Beirut, fighting in the northern city of Tripoli, and rocket attacks on Bekaa Valley towns close to the frontier.
In one of the worst spillovers, Nusra and fighters from Islamic State, an offshoot of Qaeda, attacked and briefly seized the border town of Arsal in August.
They captured several Lebanese soldiers when they pulled out. They have since killed at least three of them and are holding an unknown number of others.
(Reporting by Mariam Karouny; Editing by Jon Boyle)