The Lebanese army used its air defence systems against Syrian helicopters on Monday after they carried out a raid inside Lebanese territory, a military source told AFP.
It was the first time the Lebanese army has responded to Syrian attacks on its territory, which have multiplied as the conflict in its eastern neighbour has intensified, the source said.
"In accordance with the orders of the army command, anti-aircraft guns were fired in the direction of Syrian helicopters that bombed Khirbet Dawud near Arsal," in the area near the Syrian border, the source told AFP.
"It is the first time that the Lebanese army has used its anti-aircraft defence systems" to respond to Syrian raids, the source added.
Lebanese officials reported no casualties from the Syrian raid. It was not clear whether the retaliatory fire had hit the Syrian aircraft.
Defence Minister Fayez Ghosn said the Lebanese army was acting on the government's orders.
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The army "has clear and sufficient orders from the political authorities to respond" to such attacks, and "what happened today falls within that context".
"We respond to attacks that reach Lebanese territory," Ghosn said, adding that "since the outbreak of events in Syria, the Lebanese army, deployed along the borders, has been in full capacity and readiness to respond to anything that might affect the unity of the country".
The Lebanese army has in the past threatened to respond to cross-border fire from Syria but has not previously done so.
On June 12, it issued a rare warning to the Syrian government, saying it would respond "immediately" to any new "violation" after a raid by the army on the Arsal area, a hub of support for the rebels, which is also home to tens of thousands of Syrian refugees.
But it had not previously carried out its threat, despite repeated spillovers from the fighting over the border.
The anti-aircraft fire came a day after Lebanese President Michel Sleiman announced that Saudi Arabia had pledged $3 billion for the under-equipped army to buy French weapons.
Saudi Arabia is a key supporter of the Syrian uprising. Lebanon's Shiite Hezbollah militia, which controls much of the border region apart from the mainly Sunni Arsal district, is a leading ally of the Syrian regime.