Hezbollah fighters have killed at least four members of the al Qaida-linked Nusra Front in a rocket attack on their car in northeastern Lebanon, a security source said.
The incident took place in a restive area near the Syrian border on Tuesday (local time), just outside the Lebanese town of Arsal, which Nusra Front and Islamic State fighters overran briefly in 2014 before withdrawing after clashes with the army.
Shi'ite Hezbollah has previously targeted Sunni Islamist fighters in the area who have staged regular incursions from Lebanon's war-torn neighbour.
Nusra Front and Islamic State - themselves enemies in Syria's conflict - have in the past week clashed in the outskirts of Arsal. Nusra Front initiated the fighting with its jihadist rival by trying to capture IS positions in the area, the source said.
IS counter attacked, pushing Nusra Front out of positions it controlled east of Arsal.
The clashes killed several fighers on each side, the source said.
Nusra Front freed 16 Lebanese soldiers and policemen in December in exchange for the release of jailed Islamists. It had captured the soldiers during the Arsal incursion in 2014.
IS is believed to be still holding nine soldiers it captured.