Jerd Arsal, Lebanese-Syrian border - On the blurry border line between Syria and Lebanon lies a small stone building hidden in the rugged, mountainous terrain. It is a "surveillance centre" set up by rebels to monitor the "potential infiltration" of Shia Hezbollah fighters into the Lebanese border village of Arsal.

The cubicle is furnished with a couple of rugs, colourful mattresses and an old-fashioned diesel heater. A group of Syrian and Lebanese fighters sat by the heater, waiting for water in a teapot to boil. Their guns and walkie-talkies leaned against the room's unpainted wall.

One of the rebels, Abu Bakr, had just finished his shift patrolling the no-man's land to survey the Lebanese mountain slopes. He parked his muddied pick-up truck outside and joined the group, whose members take turns in running patrols, but also in fighting in Syrian border towns a few kilometres away against the regime of President Bashar al-Assad.

"Hezbollah has not yet made any serious attempts to infiltrate through Arsal's mountains into Syria to help Assad capture the border towns," the 25-year-old Syrian told Al Jazeera. "But if they do, we will shoot at them," he added, pouring red tea into a small glass cup.

Arsal is a lone enclave in Lebanon's eastern Bekaa Valley that is Sunni and sympathetic to the uprising against Assad. The village is surrounded by Shia villages controlled by Hezbollah. Across Arsal's border with Syria, intense clashes are raging between rebels and the Hezbollah-backed Assad troops in the strategic region of Qalamoun, which links Damascus with the central Syrian city of Homs.

The fate of Arsal



For the Sunni stronghold of Arsal, the battles in Qalamoun are crucial. Abu Bakr and his comrades call it an "existential" battle. They know that should the military campaign of the Syrian troops and Hezbollah in Qalamoun succeed, Arsal - with its 40,000 residents who mostly support the Syrian rebels and about 70,000 Syrian refugees who now live there - would be encircled by Hezbollah and Assad troops.

"If the opposition loses the Qalamoun battle, then the Syrian uprising is over and Arsal is over," Arsal's deputy mayor, Ahmad Fliti, said. He fears that Arsal would pay dearly for backing the uprising if the regime seized control of the Syrian towns bordering Arsal. He said those considered sympathetic to Syria's rebels would be rounded up and acts of reprisals would spread. "I really do not know what the fate of Arsal would be, I just know it would not be good."

With the support of Hezbollah fighters filtering into Syria from their strongholds in the towns of Hermel and Baalbek in the Bekaa Valley, Syrian government troops have managed to make swift advances in Qalamoun in recent months. However, the area bordering Arsal, which has been used as a conduit for weapons and rebels to enter Qalamoun, have so far been hard to capture.

"We have created monitoring centres all across the border between Qalamoun and Arsal in case Hezbollah thinks of sneaking in to help the Syrian regime in this battle by cutting off our supply route from Arsal," Abu Bakr said.

Last week, Syrian government troops tried to storm Flita, Abu Bakr's own village, but the soldiers were pushed back after enduring heavy losses. The village sits on Syria's highest mountain, making it easy to fend off the advancing troops. The battles forced thousands of Flita's residents to flee to Arsal. Many civilians are living in makeshift houses in no-man's land similar to Abu Bakr's monitoring centre.

Unofficial truce



But despite the heavy fighting in Syria, Sunni and Hezbollah fighters have maintained an unofficial truce on the Lebanese side of the border. "Hezbollah may resort to desperate acts and carry out a commando operation in the mountains of Arsal, like assassinate someone. But they don't need to enter into Syria from here - they already have a strong supply route from Hermel and from Baalbek," Fliti said. "The entrance of Hezbollah into Arsal would create a sectarian war between Sunnis and Shia here. This is not acceptable Lebanon-wide."

Yet the spillover of violence into Lebanon and the hardening of the sectarian divide in the Bekaa Valley cannot be hidden. Armed men from Arsal have been blamed for firing several rockets that fell in Shia border towns, and some of the suicide bombings that rocked Shia areas have allegedly been the result of car bombs prepared in Arsal.

Hezbollah, meanwhile, has set up checkpoints manned by plain-clothed men carrying automatic rifles on all roads and intersections leading to Arsal. Arsal mayor Ali al-Hujairi said the checkpoints, set up in conjunction with Lebanese army posts, are meant to "intimidate citizens".

"They are forcing people out of their cars, demanding to see their IDs and insisting on knowing their final destinations without showing any respect for the law," he said.

Hujairi told Al Jazeera that he himself is afraid of leaving the town for his own security, saying that Hezbollah members had opened fire on his car before at a checkpoint.

'There is no going back'

Umm Mohammad, a 40-year-old Lebanese, has instructed her 18-year-old son to delete the photos uploaded to his mobile phone of Syrian rebel fighters, in case Hezbollah members ask to see his phone when passing through their checkpoint on the way to his college in a nearby village. "There is a lot of tension in the street and this is scary. Every day my son returns home, I sigh in relief," she told Al Jazeera during her grocery shopping in a small Arsal store.

Umm Mohammad, like many Lebanese in Arsal, has squeezed her family into a small part of her house to host Syrian refugees. As the Qalamoun battle heats up, thousands more refugees continue to flood the village, making Umm Mohammad and other Lebanese a minority in their own town. Syrians outnumber Lebanese here almost two-to-one.

"Arsal residents have taken a side. We chose to support the opposition and we are bearing the consequences of it," Umm Mohammad said. "I don't know if it was wise, but I know that there is no going back, and we just have to submit our situation to God."

Follow Basma Atassi on Twitter: @Basma_

