A Syrian helicopter fired four rockets in and around a Lebanese town seen as close to rebels fighting the Damascus region, a security official said, in an attack condemned by President Michel Sleiman.
A rocket was fired at Aarsal, without causing casualties, and three others on the outskirts of the town in the mostly Sunni-inhabited border region with Syria, the official said.
Sleiman, in a statement, denounced "firmly the bombing that targeted the town of Aarsal today" and called for respect of Lebanon's sovereignty "and not to expose its regions to danger".
The attack took place a few hours after the capture of the Syrian town of Qusayr by regime forces backed by fighters of Lebanese Shiite militant group Hezbollah.
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Qusayr, close to the border, lies to the northeast of Aarsal.
Aarsal has been the scene of a string of deadly incursions by the Syrian army in a bid to crack down on arms smuggling. Wounded Syrians have also crossed the border into Aarsal for medical care.
The Syrian air force previously bombed the area on March 19.
The conflict in Syria has raised tensions in neighbouring Lebanon which is deeply divided between pro- and anti-regime groups.