Lebanese security officials say Syrian troops opened fire at a car that crossed illegally into northern Lebanon, killing three Lebanese men.
The officials spoke on condition of anonymity in line with regulations.
The shooting came late Tuesday as Arab League observers started a mission in Syria aimed at stopping nine months of bloody clashes during an uprising against the regime.
Some Syrians have fled to Lebanon to escape the fighting, and Syria has complained that weapons are smuggled across its borders. It was not immediately clear if the shooting was related to the uprising in Syria.
Such incidents are not uncommon on the border, since many Lebanese and Syrians cross the border illegally. Several Syrians and Lebanese were killed or wounded in recent border incidents.