* Fall of Sunni rebel town in Syria polarises Lebanon
* Rockets and car bomb hit Shi'ite towns
* Prime minister urges army to contain violence (Adds quotes, colour, background)
By Alexander Dziadosz
AL-LABWA, Lebanon, March 17 (Reuters) - Lebanese gunmen sprang into action and sirens blared on Monday as rockets struck a mainly Shi'ite town near the Syrian border where authorities are struggling to contain sectarian violence fuelled by a Syrian army offensive across the frontier.
The rocket attack on al-Labwa was the latest strike on a Shi'ite target inside Lebanon after Syrian President Bashar al-Assad's forces and their Lebanese Hezbollah allies recaptured the border town of Yabroud from Sunni Muslim rebels on Sunday.
The rebel defeat at Yabroud sent a stream of refugees and fighters across the border towards the Lebanese Bekaa Valley town of Arsal, and was followed hours later by a suicide car bombing against a local stronghold of Shi'ite Hezbollah.
The border area has been steadily sucked into Syria's three-year-old conflict as Syrian troops and jets targeted rebel bases on the frontier and suspected Syrian rebels fired rockets at Shi'ite towns to punish Hezbollah for supporting Assad.
But the rebel loss of Yabroud could exacerbate sectarian tensions across Lebanon and the flight of 2,000 defeated rebels - some of them into Lebanese territory - would further destabilise the already volatile Bekaa Valley.
"Everyone is scared about the future. They're scared that after the fall of Yabroud the gunmen who fled might make trouble here," said Talal Mohieddin, a 35-year-old shop owner in the town of al-Ain near the border.
"No one is leaving their house. Everyone is on guard," he said, gesturing to the town's empty streets and shuttered concrete homes and stores.
Lebanese Prime Minister Tammam Salam met army chief General Jean Kahwaji on Monday and called on the military to "take all necessary measures to control the situation in Bekaa's border areas", a statement from his office said.
When the rockets struck al-Labwa, a mainly Shi'ite town about eight km (five miles) west of the Sunni town of Arsal, gunmen took up positions on the street and pointed rifles east toward the mountainous border with Syria. Some leapt into cars and sped away as emergency sirens blared.
The army said in a statement a total of four rockets had hit the area, wounding one person. Earlier in the day, security forces blew up a suspected car bomb and combed the border town of Fakeha for those who had planted it.
TRIUMPH AND TREPIDATION
The attack on al-Labwa followed a suicide bombing which killed three people in the nearby town of Nabi Osmane on Sunday. Two radical Islamist groups with suspected ties to Sunni al Qaeda militants in Syria claimed responsibility.
At the site of the blast, yellow Hezbollah banners were flying on Monday. "Dear criminals, our blood is stronger than your terror," read one of them, next to the group's logo.
The blast blew apart buildings in the area, including a barber shop where the twisted remains of a barber's chair were visible through the door. A damaged grey Mercedes was parked in the road near the twisted, charred remains of another car.
One person was killed in the same town on Saturday after several rockets were fired from near Arsal.
As bulldozers cleared rubble and shattered glass from the blast site on Monday, bystanders derided what they called "terrorists," the standard term used by Syria's government and Hezbollah to describe Syrian rebels.
"These terrorists have no religion, they know no god," said Mona Taiya, a 48-year-old resident who, like many in the area, works in agriculture.
She voiced doubts that Lebanon's army could restore security on its own, saying that task would fall largely to Hezbollah. "The Lebanese army helps, but only as much as it can," she said.
Yabroud was the last rebel stronghold on the Syrian side of the border and its fall ignited open celebrations in Beirut's southern Shi'ite suburbs - mixed with fear of revenge attacks.
More than 100 youths on motorcycles paraded through the district on Sunday, waving Hezbollah flags and hooting their horns, and sheep were slaughtered in front of a mosque.
Hours later, however, Hezbollah members deployed in the streets after the Bekaa suicide bombing. Soldiers blocked off entrances to the suburbs and there were lengthy queues at the few open checkpoints, where cars were thoroughly searched.
Despite the tensions, residents near the border on Monday dismissed the risk of Syria's conflict leading to a new war in Lebanon, portraying it instead as a security problem for the authorities.
Akram Shammas, a 43-year-old supermarket owner in al-Labwa, said he was anticipating more car bombs in Lebanon after the fall of Yabroud, but that full-scale war was unlikely.
"If a civil war was going to happen, it would have happened after the battle of Qusair," he said, referring to another rebel-held town near Lebanon that Assad's forces and Hezbollah captured last year.
Earlier on Monday, the army blew up an explosives-laden car about five km north of Sunday's suicide attack. The twisted and charred remains of the car lay in a field of almond trees on a hillside on the outskirts of the small town of Fakeha.
Army humvees mounted with machine guns rolled along the crest of the hill and Lebanese soldiers in fatigues patrolled through the fields and the town, taking up positions along roadsides looking for the men who had been driving the car.
Access from al-Labwa to Arsal was blocked, possibly to prevent Syrian rebels who may have crossed into Arsal from clashing with local Shi'ites in al-Labwa.
"The road into and out of Arsal is cut," Arsal mayor Ali al-Hujeiri told Reuters, adding that more than 400 families had arrived in the town over the last 48 hours.
He said only a handful of the 100 wounded people who were reportedly treated in an Arsal field hospital were rebels, but that other fighters may have taken refuge in the rugged and remote border region surrounding the town.
In Tripoli, the army clashed overnight with fighters who fired rockets at military posts in the northern coastal city, security sources said. Twelve people have been killed in four days of fighting stoked by tensions between Sunnis and minority Alawites, from the same faith as the Syrian president.
In his meeting with the army chief, Prime Minister Salam also instructed the military "to adopt zero tolerance" towards anyone threatening security in Tripoli, Lebanon's second city. (Additional reporting by Dominic Evans; Editing by Alistair Lyon)