Clashes following an attack by "a group of terrorists" led to the killing of six soldiers and the wounding of a seventh, an army statement said.
Earlier, a military source said seven soldiers had died in the clashes in the desert region of Ras Baalbek.
The ambush came as news emerged that Lebanon detained in the nearby town of Arsal a wife and young son of Abu Bakr al-Baghdadi, the head of the jihadist Islamic State (IS) group that has seized large parts of Syria and Iraq.
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Last month a roadside bomb wounded three Lebanese soldiers near Arsal, where troops in August engaged in fierce combat with jihadists who streamed across the border.
Fighting ended with a truce mediated by clerics, but the jihadists took with them 30 Lebanese army and police hostages.
Three have since been executed, and Qatari-led efforts to free the rest have so far yielded no concrete result.
Lebanon is sharply divided over the war in Syria, and has regularly seen deadly battles linked to the conflict next door.
Most Sunnis support the revolt seeking the ouster of Syrian President Bashar al-Assad, while Shiites generally back Damascus and its ally Hezbollah.