Lebanon's army said it will immediately respond to any further cross-border attacks by the Syrian military after a helicopter gunship attacked the eastern town of Arsal on Wednesday.
"Army units deployed in the (Arsal) area took the necessary defensive measures to respond immediately to any similar violations," a Lebanese army statement said, in a rare warning against the regime of Syrian President Bashar al-Assad.
Earlier Wednesday, a helicopter raided Arsal, the majority of whose residents support the anti-Assad revolt in neighbouring Syria, for the first time in the nearly 27-month conflict.
At 1:30 pm (1030 GMT), "a Syrian helicopter gunship crossed the border... and launched two rockets from a distance at the (Arsal) town centre, injuring one person and causing material damage", the army said.
Damascus dominated Lebanon politically and military for 30 years until 2005, and continues to exert significant influence through its allies in the Mediterranean country.
Lebanon's poorly equipped army coordinates closely with the Syrian military.
Arsal is sensitive because it is located just 12 kilometres (7.5 miles) from the border with Syria.
Experts say it has been used as a conduit for weapons and rebel fighters to enter into Syria, while it has also served as a refuge for people fleeing the conflict into Lebanon.
Dozens of people wounded in fighting in Qusayr -- most of them rebel fighters -- flocked to Arsal for treatment last week as the Syrian army and Lebanese Shiite movement Hezbollah overran the former insurgent bastion.