Syrian children inside Zaiytouni Mosque in Arsal, Lebanon where around 100 refugees have sought shelter on November 24, 2013. (Loveday Morris/The Washington Post)

Its dusty streets lined with cars bearing Syrian license plates, this Lebanese mountain town has long felt as much Syrian as Lebanese. But as Bashar al-Assad’s army squeezes rebel-held towns just across the border, 20,000 new arrivals have left locals significantly outnumbered and forced Lebanon to open its first official transit camp for Syrian refugees.

Many arrive in the border town with little more than the clothes on their backs, packing into wedding halls and mosques or sleeping in cars while awaiting tents in the newly organized camp.

The influx comes as the Syrian army, backed by forces from the Lebanese Shiite militant group Hezbollah, moves to secure towns in the Qalamoun region across the border. It started as government forces took Qara, a highway town dotted with car mechanic shops on the road from Homs to the capital, Damascus, a fortnight ago. The town emptied, and a convoy of thousands left for Lebanon, the winding dirt road across the mountains backed up with cars and trucks. On Thursday, Deir Attiyeh, a few miles farther south on the highway, was retaken by government forces after being seized by rebels days earlier, while nearby Nabk remained surrounded.

Officials in Arsal say they fear that the continuing instability on the town’s doorstep means there will be no easing in the flow of families moving across the border.

Dania, 22, who declined to give her last name, was taking shelter in Arsal’s Zaytouni Mosque with her husband and two children. When night falls, about 100 people gather in the mosque to sleep, the men separated from the women and children by an old carpet strung up across the prayer hall.

Dania struggled to count how many times she has had to flee violence in the past two years — first from her home in Homs to a safer neighborhood in the city, then to the Damascus suburbs and on to Qara before being forced out to Arsal two weeks ago.

“We cannot bear any more,” she said. “We feel chased. We just want to protect our children.”

She expects to be moved to the transit camp and is bracing to spend the winter in a tent.

Although unofficial encampments have sprung up across the country as more than 800,000 refugees fled to Lebanon over the past two years, the country had held back until now from establishing formal camps such as those in Jordan and Turkey. With Palestinian camps off limits for the Lebanese army, and blamed by some for sparking the Lebanese civil war, setting up refuges for Syrians is a politically fraught step.

The Arsal camp, which currently houses 70 families and has space for 250, is the first to be authorized by the Lebanese Ministry of Social Affairs, said Dana Sleiman, a spokeswoman for the U.N. High Commissioner for Refugees. Sleiman stressed, however, that the refuge is “temporary” and will serve only as a transit camp while alternative accommodation is being sought.

Still, Arsal’s deputy mayor, Ahmed Hassan al-Fliti, said there is nowhere else for people to go. Construction, largely illegal, is booming, he said, but despite the addition of 4,000 new houses since the beginning of the Syrian war, almost doubling the town’s size, demand is outpacing supply. According to municipality figures, the number of Syrian refugees in the town is now more than 60,000, in addition to just under 40,000 residents.

And few expect the situation to ease. For the Syrian government, the Qalamoun offensive is a means to securing the Homs-to-Damascus highway and is helping sever rebel supply lines to both cities.

Majd Mohammed, 26, a fighter who spent the past two years in Qara and was wounded by mortar fire a day after Syrian government troops entered in mid-November, said weapons for the countryside surrounding Homs and the Damascus suburbs were funneled through the town.

“Qalamoun is important first, because they want to control the highway, and second, because they want to stop the weapons smuggling,” he said as he recovered in an Arsal clinic.

Fliti said that at least 500 tents are needed within the week just for families already in the town. Many fear moving on to other parts of Lebanon. Sunni Arsal is surrounded by largely Shiite villages in the Bekaa Valley, where there is a groundswell of support for Hezbollah.

With Syrian troops on one side and Hezbollah on the other, fleeing Sunni Syrians say they feel penned in. Syrian forces sporadically fire rockets at the outskirts of the town, which Fliti described as “political messages.”

Lebanese authorities increasingly view the town as a dangerous powder keg. An Arsal native was named in the investigation of an August bombing in a Hezbollah stronghold in southern Beirut. A week ago, a car packed with explosives was found in a Shiite village a few miles from the town, while aid workers were removed from Arsal because of car-bomb threats.

“Before, we had 40,000 people and 40,000 refugees,” said Qassim Zain, head of the clinic. “Now the balance is shifting, and there are fears there could be more problems.

“But there’s no solution,” he added. “Every village they clear, they will come to Lebanon. When this happens, Lebanon will be drowned.”

Fliti fears that the town of Yabroud, which like many of the towns in Qalamoun is already swollen with the internally displaced, will be next to fall.

“If we got 20,000 from Qara, we’d expect at least 40,000 from Yabroud,” he said. “It’s a human time bomb here.”

Suzan Haidamous in Arsal and Ahmed Ramadan in Beirut contributed to this report.