Lebanese troops have fired at Syrian warplanes violating its airspace, for what is thought to be the first time since the conflict in Syria began.
Lebanon's National News Agency said the army had responded to a raid on Khirbet Daoud, near Arsal in the Bekaa Valley.
Syrian government forces have fired into Lebanon in the past, targeting rebels sheltering over the border.
The Lebanese authorities had until now not responded militarily, hoping they would not be dragged into the war.
Arsal is predominantly Sunni and its residents have been broadly supportive of the Sunni-dominated uprising against Syrian President Bashar al-Assad, whose Alawite sect is an offshoot of Shia Islam.
The north-eastern town has been flooded with refugees since the Syrian military launched an offensive in the Qalamoun mountains last month.
Some 20,000 people have settled in makeshift camps, as Syrian troops backed by members of the militant Lebanese Shia Islamist movement Hezbollah have sought to cut rebel cross-border supply routes.