By Laila Bassam
BEIRUT (Reuters) - Gunmen killed three Lebanese soldiers at an army checkpoint in the eastern Bekaa Valley on Tuesday before fleeing towards the Syrian border, Lebanese officials said.
It was not clear who carried out the attack, the latest incident in a frontier region which has been increasingly drawn into the violence in neighboring Syria.
The Syrian civil war has divided Lebanon, with most Lebanese Shi'ites supporting Syrian President Bashar al-Assad and many Sunnis backing his Sunni rebel foes, putting the Lebanese army under extra pressure to keep a lid on sectarian tensions.
Most Sunni groups in northern and eastern Lebanon blame the army for hindering their efforts to support rebels in Syria with guns and fighters and at the same time failing stop the Shi'ite Hezbollah militant group from sending fighters to support Assad.
Tuesday's shooting took place before dawn near the town of Arsal, in an area used by Syrian rebels and their Lebanese backers to smuggle arms and fighters into Syria.
The military was searching for the gunmen, who may have fled into neighboring Syria, Defense Minister Fayez Ghosn said.
President Michel Suleiman condemned the killings which he said were "part of a series of criminal terrorist acts trying to spread turmoil in the country". Hezbollah also called them a "terrorist crime" which threatened the whole country.
Several rockets have been fired into the mainly Shi'ite town of Hermel in the past 24 hours, about 30 km (20 miles) north of Arsal. One of them killed a woman and wounded two people, the army said.
The violence in Syria, where more than 80,000 people have been killed in 26 months, has spilled into Lebanon with rising frequency, raising 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.
Hezbollah has been battling alongside Assad's forces to drive rebels from the Syrian border town of Qusair, while many pro-rebel Sunni gunmen 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 fuelled partly by tensions over the Qusair battle, and in the capital two rockets were fired on Sunday at Hezbollah's southern Beirut stronghold.
The rocket attack followed a speech by Hezbollah's leader Sayyed 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.
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.
Brigadier Selim Idris, head of Western-backed rebel front, said on Tuesday that unless Hezbollah attacks were halted within 24 hours, the rebels would "chase down" Hezbollah.
"We will chase them all the way to hell if a decision is not taken to stop Hezbollah's attack on Syrian land," he told Al Arabiya Television.
The army has confronted gunmen before in the Bekaa Valley town of Arsal. In February, at least two soldiers and two gunmen were killed in a shootout after the army entered the area to arrested a suspected member of the al Qaeda-linked Nusra Front.