The four rain-filled bomb craters all visible within 100 yards of Mahmoud Ismael’s house starkly illustrate how Lebanon’s northern border has become an active frontline in Syria’s civil war, drawing in rival Lebanese Shiite and Sunni factions.

A fifth shell had struck the edge of the roof, knocking out chunks of concrete and sending heavy steel shrapnel scything into the cement parapet and the soft earth below.

“It was a terrifying night. We all thought we would be killed,” says Mr. Ismael, surveying the damage.

The Lebanese government, which follows a policy of neutrality towards the war in Syria, has found itself almost powerless to prevent pockets of north Lebanon becoming either bastions of support for the Syrian regime or de facto safe havens for the armed Syrian opposition.

Tensions between Lebanon’s Sunni and Shiite communities have been running high for several years. But they have been aggravated further by the increasingly sectarian nature of the conflict in Syria, which has pitted the majority Sunni opposition against the Alawite minority, a subsect of Shiite Islam which forms the backbone of the regime of President Bashar al-Assad.

Nourat al-Tahta, like other Sunni-populated villages along the border in the northern Akkar province, is deeply supportive of the Syrian revolution and shelters refugees and Free Syrian Army militants alike. The villages in the area have been subjected to Syrian artillery shelling on a near nightly basis since May. The shelling is intended to hit FSA members who slip across the border into Syria at night as well as to punish those Lebanese who provide assistance and a safe haven for the militants.

Until recently, it was confined mainly to the scrubland outside the village, but in the past week, the bombardments have focused more closely on inhabited areas.

“If it carries on like this we will have to leave,” Ismael says.

According to a Lebanese member of the FSA who lives in the area, Nourat al-Tahta was the starting point three weeks ago for 20 Lebanese Sunni volunteers who set out to cross the border and join a rebel group. The volunteers fell into an ambush on the Syrian side of the border and 14 of them were killed, according to the militant. The shelling, he said, was the Syrian regime’s punishment on the village.

The Syrian shelling and clandestine FSA activities underline how little state control exists in the northern Akkar. The Lebanese Army has sent some additional reinforcements to the border, but its ability to contain the violence is limited. Returning artillery fire into Syria is politically impossible for the Lebanese army, while chasing after FSA militants operating in Lebanon risks incurring the anger of local Lebanese Sunnis.

Further east along the border, on the other side of 6,500-foot forested mountains that last week were lashed by torrential rain and capped in snow, lies the stony flatlands of the Shiite-populated northern Bekaa Valley, an area of strong support for the Lebanese Shiite militant group Hezbollah, an ally of the Assad regime.

On a recent afternoon, the steady crump of artillery explosions and the sharp pop of outgoing mortar rounds just inside Syria reverberated through the border town of Qasr as Hezbollah vehicles – SUVs with tinted windows and no license plates – raced through the narrow potholed streets of the town.

Some 25 small villages populated by Lebanese Shiites lie just across the border from Qasr, in Syria. They have been the focus of repeated clashes in recent months, pitting the FSA against Syrian troops allegedly backed by Hezbollah militants.

The fighting is expected to grow more intense in the coming days because the FSA’s Omar al-Farouq brigade, one of the largest and most successful Syrian rebel units, has redeployed much of its manpower from the border area opposite north Lebanon to Damascus, according to Syrian opposition sources. Lebanese and Syrian FSA militants, who had been resting in the Bekaa, are said to be heading into Syria to reinforce the rebels’ depleted ranks.

The residents of Qasr believe that the Syrian rebels are seeking to empty the Shiite villages just over the border to create a corridor linking Sunni areas to facilitate movement across the top of north Lebanon.

“Those villages won’t go down easily. They will defend them to the last bullet. They are willing to fight to the end,” Abu Ali, a member of Qasr’s municipality, says.

The residents tell lurid tales of atrocities committed by the rebels whom they accuse of being Islamic extremists and many of whom they say are not even Syrians.

“There is no Free Syrian Army, they are all Salafists who are attacking us and robbing our homes,” says Minjad al-Haq, a resident of Safsafah, one of the Shiite villages inside Syria who moved across the border in September to escape the fighting. “They are decapitating their prisoners. They say Allahu Akhbar three times then cut off their heads.”

The emergence and growth of radical Islamist groups in Syria – such as Jabhat al-Nusra, which was recently proscribed by the US as a terrorist organization – and indications of Sunni radicalization in Lebanon have unnerved Lebanese Shiites who worry they could be targeted by triumphant Sunnis once the Assad regime falls. Some of those Sunni militants exist in the northeast corner of the Bekaa Valley, which has become a safe haven for the FSA and a conduit for militants to slip into Syria. All that separates Hezbollah and its Shiite supporters in the northwest pocket of the Bekaa from their Sunni FSA enemies in the northeast corner of the valley is a no-man's land of flat, stony earth.

Although Hezbollah has fought the FSA just north of the border inside Syria, a tense calm exists south of the frontier.

“The Shiites in Syria are in a defensive mode and the Sunnis are in attack mode,” says Abu Ali. “But if the Sunnis attack us here [inside Lebanon] we will attack them here.”

But the sense that the Assad regime’s days are numbered is emboldening some Sunnis to cast their mind toward the next conflict.

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Khaled, a portly Lebanese Salafist from the Bekaa Valley who has fought with the FSA for 18 months, predicted that the Assad regime would collapse within eight weeks.

“When we are done there, we will come after Hezbollah here,” he says. “We are going to finish them completely. The Free Syrian Army will come and clean Lebanon of Hezbollah then leave, just like we helped them clean Syria of Assad.”