Lebanon's army warned it will hit back against any new attacks from Syria after a helicopter gunship struck an eastern town on Wednesday, ratcheting up tensions ahead of US-British talks on the conflict.
AFP reported that the escalation came hours after the emergence of video footage showing the mutilated corpse of one of some 60 Shiites killed in the latest outbreak of worsening sectarian violence in Syria.
A Syrian helicopter gunship fired two rockets at the center of Arsal, a Lebanese town populated mostly by Sunni Muslims, wounding one person, Lebanon's army said.
In a rare warning against the regime of Syrian President Bashar al-Assad, it said Lebanese troops "took the necessary defensive measures to respond immediately to any similar violations".
Lebanon's President Michel Sleiman called the attack "a violation of Lebanon's sovereignty" and said Beirut has the right to take steps to defend itself and to "submit a complaint" to the UN and the Arab League.
Most residents of Arsal, in the hills just 12 kilometers (7.5 miles) from the border with Syria, support the Sunni-led uprising against the Assad regime.
Lebanon's poorly equipped army normally coordinates closely with the Syrian military.
Damascus dominated Lebanon politically and military for 30 years until 2005, and still exerts significant influence through its allies there.
Arsal has been used as a conduit for weapons and rebels to enter Syria, while also serving as a refuge for people fleeing the conflict.
AFP noted that dozens of people wounded in fighting in Qusayr -- mostly rebels – flocked to Arsal last week as Syria's army and fighters from the Lebanese Shiite movement Hizbullah overran the former insurgent bastion.
In a new macabre episode, Sunni Islamist rebels celebrated the killing of some 60 Shiites, mostly pro-regime fighters, according to amateur video distributed by the Syrian Observatory for Human Rights.
"Look Shiites, this is how you will end up, you dogs," cries one man shown in footage filmed in Hatlah village, in the eastern Syrian province of Deir Ezzor.
The rebels assaulted Hatlah in retaliation after Shiite fighters from the village attacked a rebel position, killing two insurgents, said the Observatory.
Boosted by their victory in Qusayr, regime troops advanced on parts of the nearby city of Homs, as they prepared to assault Aleppo in the north, the Observatory said.
France, meanwhile, urged the international community to stop the Assad regime.
"We need to re-balance things because over the past few weeks the troops of Bashar al-Assad and especially Hizbullah and the Iranians, along with Russian arms, have gained considerable ground," said Foreign Minister Laurent Fabius.
He did not elaborate.
On Tuesday, Fabius accused Assad of using "chemical weapons in an outrageous manner".