The official says Lebanese troops found the bodies of three missing soldiers today on the outskirts of the northeastern Lebanese village of Ras Baalbek.
Late yesterday, an army statement said another five soldiers were killed in the fighting.
The official, who spoke on condition of anonymity in line with regulations, said the attackers belong to the Islamic State group that controls large parts of Syria.
In August, al-Qaida-linked Nusra Front and Islamic State group fighters conducted a joint raid in the border town of Arsal and captured some two dozen soldiers. Since then, they have killed four of the soldiers.