The Lebanese Army brought in reinforcements on Monday as part of an advance into a border town attacked by Syrian Islamists over the weekend.
Gunmen identified as members of the al-Nusra Front and the Islamic state have already seized parts of Syria and Iraq.
On Sunday they killed at least 14 Lebanese troops and injured scores more. Twenty-two others have been reported as missing.
The fighting erupted after Lebanese security forces arrested a Syrian Islamist rebel commander, Emad Jumaa, on Saturday.
Lebanon’s Prime Minister Tammam Salam condemned the killings:
“There are no political solutions with these extremists who are tampering with Arab societies under oppressive alien religious slogans and who want to move sick practices into Lebanon,“he said.
The attack on Arsal was the first major cross border incursion by hardline Sunni militants who are becoming more influential in Syria’s civil war.
Since the uprising against Syrian President Bashar al Assad, the town of Arsal has become a refuge for fleeing Syrian families.
During Monday’s Lebanese military advance, army trucks brought out hundreds of civilians who had been prevented from leaving by the fighting.
Lebanon is still rebuilding from its own 1975-1990 civil war and has been hit by violence linked to the Syrian conflict including rocket attacks, suicide bombings and gun battles.
There is growing concern in the county that the Sunni – Shi’ite conflict in its neighbour could destabilise Lebanon by inflaming its own sectarian tensions.