Lebanon's army warned it will hit back against any new attacks from Syria after a helicopter gunship struck an eastern town today, ratcheting up tensions ahead of US-British talks on the conflict.
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 centre 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 kilometres from the border with Syria, support the Sunni-led uprising against the Assad regime.
The majority of Syria's population is Sunni but it has been ruled by more than 40 years by the Assad clan from the Alawite community, an offshoot of Shiite Islam.
The Syria conflict erupted in March 2011 following a bloody regime crackdown on Arab Spring-inspired democracy protests.
Since then, there have been several spillover attacks in Lebanon involving both sides in Syria, but today's army statement was the first of its kind in the nearly 27-month conflict.
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.
