Armed men killed three Lebanese soldiers at an army checkpoint in the eastern Bekaa Valley then fled towards the Syrian border, Lebanese officials say.
It was not clear who carried out Tuesday's attack, which is the latest incident in a frontier region which has been increasingly drawn into the violence in neighbouring Syria.
The shooting took place before dawn near Arsal, in an area used by Syrian rebels and their Lebanese backers to smuggle arms and fighters into Syria.
In February, four Lebanese soldiers and two fighters were killed in a gun battle near Arsal.
Lebanon is sharply divided over the war in Syria and violence and the majority Sunni Muslim village of Arsal is a particular flashpoint as refugees from the uprising and fighters hostile to the Syrian government of President Bashar al-Assad traverse the border.
Al Jazeera's Rula Amin, reporting from Beirut, said as the conflict continues the division between the allies of Bashar al-Assad and the allies of the rebels in Lebanon seems to be getting worse.
The situation has put the Lebanese army under extra pressure to keep a lid on sectarian tensions.
"Soldiers at the checkpoint confronted the attackers and a clash ensued which resulted in the martyrdom of three soldiers," an army statement said.
The military was searching for the assailants, who may have fled into neighbouring Syria, Fayez Ghosn, Lebanon's defence minister, said after Tuesday's attack.
Three mortar shells fired from Syria on Monday killed at least one woman and wounded two people near the eastern Lebanese town of Hermel, about 30km north of Arsal, the Lebanese army said.
Hermel is a bastion of Hezbollah, the armed Lebanese Shia group.
Most Sunni groups in northern and eastern Lebanon blame the country's army for hindering their efforts to support rebels in Syria with guns and fighters and at the same time failing stop Hezbollah from sending fighters to support Assad.
Hezbollah has been battling alongside Assad's forces to drive rebels from the Syrian border town of Qusayr, while many pro-rebel Sunni fighters have slipped across to join the uprising.
In Lebanon's northern city of Tripoli at least 25 people were killed last week in street fighting prompted partly by tensions over the Qusayr battle, and two rockets were fired on Sunday at Hezbollah's southern Beirut stronghold.
The rocket attack followed a speech by Hezbollah's leader Hassan Nasrallah, who promised victory in the group's fight to defend Assad.
Syria has served as Hezbollah's conduit for weapons supplied mostly by Iran for the past three decades.
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No one claimed responsibility for the rockets that hit southern Beirut, but it was widely assumed to be a response to Nasrallah's speech by Syrian rebels who have threatened to take the fight into Lebanon unless Hezbollah keeps out of Syria.
Nasrallah pledged in a speech that Hezbollah would turn the tide of the conflict in Assad's favour, and stay as long as necessary to do so.
The violence in Syria, where 80,000 people have been killed in 26 months, has spilled into Lebanon with rising frequency.
This in turn has raised fears for the fate of a small nation that lost anywhere between 100,000 and 150,000 dead in its own 1975-90 civil war.