The source said it was unclear which group the militants belonged to. They were driving near the outskirts of the town of Arsal, the source said, without providing further details.
Islamist groups including al Qaeda's Nusra Front have a presence in western Syria, including near the border with Lebanon, and there are have been numerous incursions by Islamists reported in the porous border region.
Syria's four-year conflict has spilled over into its smaller neighbour, which is still rebuilding after its own 15-year civil war. There have been clashes inside Lebanon between gunmen loyal to opposing sides in the Syrian conflict, as well as strikes on the army and cross-border attacks by Syrian rebels.
The army fought several days of deadly battles last year with insurgent groups including Islamic State and Nusra Front when they staged an incursion into Arsal.