The move suggests Beirut is trying to enforce greater respect for its borders in the hope of slowing the expansion of the conflict into Lebanon, where it has exacerbated sectarian tensions and prompted shadowy groups to conduct attacks that have killed dozens this year.
Lebanese officials said the military fired anti-aircraft guns at two Syrian helicopters after they fired four projectiles in a mountainous area close to the eastern Lebanese town of Arsal.
A Lebanese military official could not confirm the report, but said the army has orders to shoot anything - planes, tanks or troops - that violated Lebanese territory.
There was no comment from the Syrian government.
Meanwhile, a UN official said at least 15 people have died of hunger-related illnesses in a besieged area of Damascus over the past four months.
The UN's Relief and Works Agency that supports Palestinian refugees had until recently shipped food into the area, but has not been able to do so since September, said the organisation's Chris Gunness.
He said at least five people died over the weekend, but 10 people had previously passed away in the months before. The dead include men, women and children.