The Lebanese army has fired on Syrian aircraft that violated the country's airspace, according to the country's security officials.
Lebanon's National News Agency said on Monday the army had responded to a raid on Khirbet Daoud, near the eastern town of Arsal in the Bekaa Valley, close to the border with Syria.
"In accordance with the orders of the army command, anti-aircraft guns were fired in the direction of Syrian helicopters that bombed Khirbet Dawud," a security source told the AFP news agency.
"It is the first time that the Lebanese army has used its anti-aircraft defence systems" to response to Syrian raids.
Lebanese officials, who spoke on condition of anonymity because they were not authorised to comment publicly, said the military fired anti-aircraft guns at two Syrian helicopters after they fired four missiles into the mountainous, barren area.
Syrian aircraft have frequently conducted strikes near the frontier, sometimes hitting Lebanese territory.
In-depth coverage of escalating violence across Syria
Lebanon had protested but not responded with force.
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.
There was no comment from the Syrian government.
Local security officials said the Syrians were chasing rebels who were trying to sneak into Lebanon.
Communities on the Lebanese side of the border dominated by Sunni Muslims have become safe havens for rebels battling the rule of Syrian President Bashar al-Assad.
Monday's reported army action comes after Lebanese President Michel Suleiman announced that Saudi Arabia had pledged $3bn for Lebanon's army, calling it the largest grant ever given to the country's armed forces.
The pledge occurred as Lebanon held a funeral for Mohamad Chatah, the former finance minister, amid rising tensions over who might have killed him.
French President Francois Hollande, on a visit to Saudi Arabia where he met King Abdullah, said France would supply weapons to the Lebanese army if it was asked to do so.
"France has equipped the Lebanese army for a while up until recently and we will readily answer any solicitation," Hollande said in Riyadh
"If demands are made to us we will satisfy them."