Lebanese army soldiers help Syrian refugees out of a truck after they were evacuated from the Sunni Muslim border town of Arsal in the country’s eastern Bekaa Valley on Monday. (Hassan Abdallah/Reuters)

Lebanese troops battled rebels from Syria for control of a border town Monday, the deadliest challenge in years for the armed forces of this tiny country, whose own sectarian tensions could boil over from the incursion.

Dozens of armored personnel carriers, tanks and elite troops deployed to surround the town of Arsal, about 55 miles from the capital, Beirut. Meanwhile, thousands of Lebanese civilians and Syrian refugees fled.

The fighting in Arsal, which began Saturday, marks the first time that Sunni militants battling Syrian President Bashar al-Assad have carried out a large-scale incursion into Lebanon. The clashes have killed 17 soldiers, while 22 others remain missing, authorities said.

The civilian exodus came early Monday during a lull in the fighting. A few hours later, there was heavy army bombardment around Arsal.

It remains unclear what allegiances the Syrian rebels who seized Arsal have. Their attack comes after the Lebanese army said Saturday that its troops had detained a man who identified himself as a member of the al-Qaeda-linked Jabhat al-Nusra — one of the most powerful Syrian rebel groups.

A Lebanese family waits to depart in a minibus on the outskirts of Arsal on Monday. Civilians fled the city during a lull in fighting between Lebanese troops and Syrian militants. (Bilal Hussein/AP)

Former Lebanese prime minister Saad Hariri, a Sunni leader with a large following, has accused al-Qaeda-linked rebel groups in Syria of taking Arsal hostage.

The clashes in Arsal, a predominantly Sunni town of 40,000 whose population has almost tripled because of the presence of Syrian refugees and rebels, could worsen sectarian tensions in Lebanon. The town is wedged between Syrian government-controlled territory and Lebanese Shiite villages sympathetic to Lebanon’s premier Shiite militia, Hezbollah.

The Syrian government has seized nearly all of the strategic Qalamoun region bordering Arsal with the help of Hezbollah. On Monday, some Hezbollah members were seen around Labweh, a town near Arsal in Lebanon’s Bekaa Valley, though it was not clear whether they were taking part in the fighting.

A senior Hezbollah official vowed Monday to support the Lebanese army against the militants.

“Whoever threatens to divide the army . . . we say to them that neither Lebanon nor the Bekaa [Valley] is Mosul,” said Sheik Mohammad Yazbek, referring to Iraq’s second-largest city, which has been seized by the Islamic State, which also is fighting in Syria.

But it is unlikely that the militants raided Arsal to gain Lebanese territory, said Aram Nerguizian, a senior fellow at the Center for Strategic and International Studies. He said Arsal was surrounded by Christian, Shiite and even Sunni areas hostile to Islamist militants.

“This would be the definition of a very precarious deployment,” Nerguizian said.

Meanwhile, in Syria, the Islamic State fatally shot and stabbed a family of seven in a village near the central town of Salamiyah, the Britain-based Syrian Observatory for Human Rights reported.

Syrian state media also reported the attack. The family belonged to the Ismaili sect, a branch of Shiite Islam. Many of the extremist Sunni groups fighting in Syria and Iraq view all other sects of Islam as heretical.

— Associated Press