A group of gunmen killed three Lebanese soldiers near the Syrian border overnight, the army said on Tuesday, in the deadliest such attack since the start of Syria's uprising.
Lebanese President Michel Sleiman condemned the "terrorist" attack and called on the army to strike the perpetrators with an "iron fist", in remarks echoed by the militant Shiite movement Hezbollah.
"Around 3:30 am, an army position in the Wadi Hmayyed-Arsal was attacked by an armed group in a black Jeep," said the army.
"The soldiers at the checkpoint faced off against the attackers, clashes ensued and three soldiers were killed," it said in a statement.
"Searches are continuing in the area for the armed men who escaped into the neighbouring field," it added without blaming any party for the attack.
A security source earlier told AFP the attack had taken place east of Arsal, a town in northeastern Lebanon where most residents support the Syrian uprising.
The attack is "part of a series of terrorist, criminal acts that seek to instigate discord in the country," the Lebanese president said.
Sleiman also called on the army to strike with "a fist of iron" anyone behind the attack.
Hezbollah, whose militants are fighting in Syria alongside regular forces battling anti-regime rebels, said the "hand of terrorism and evil" is responsible for the attack on the army and paid tribute to the troops.
Outgoing Prime Minister Najib Mikati denounced the "aggression" and talked of the "difficult circumstances" through which Lebanon was passing.
Since the start of the rebellion against Syrian President Bashar al-Assad more than two years ago there have been outbreaks of deadly violence on the Lebanese border.
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Two Lebanese soldiers were killed in Arsal in February when an army patrol was ambushed while chasing a wanted man. The incident sharply raised tension between residents and the army.
Lebanon is deeply divided over the war in Syria, and the majority Sunni Muslim village of Arsal is a particular flashpoint as refugees and fighters hostile to the Assad regime traverse the border.
Three rockets fired from Syria on Monday killed at least one woman near the eastern Lebanese town of Hermel, a Hezbollah bastion, a security official told AFP.
On Tuesday, six more rockets hit the area. The first lightly slammed into a house on the outskirts of the town, destroying it and wounding two people, while the others fell inside Hermel wounding five people, including a girl, the source said.
Hezbollah militants are engaged in fighting rebels in the central Syrian town of Qusayr, alongside regime forces, and has lost dozens of men in the battles.
Since the start of the Syria conflict in March 2011, a number of rockets and mortar shells have struck Lebanese territory.
On two occasions, the rebel Free Syrian Army claimed responsibility for the attacks in response to Hezbollah's involvement.
Hezbollah chief Hassan Nasrallah has promised "victory" to his supporters in Syria.
On Monday, rockets fell for the first time in the southern suburbs of Beirut, a Hezbollah stronghold.
The Syrian crisis has also spread to northern Lebanon, where fighting last week between Alawite supporters and Sunni opponents of the Assad regime has killed 31 people.
The Syrian regime is led by the Alawite minority community, an offshoot of Shiite Islam, while the majority of the country's population are Sunni Muslims.