Updated

The Lebanese army sent commandos to the tense border area with Syria on Monday, bracing for another spillover of the conflict next door as rebels there continued to flee into Lebanon after the fall of one of their strongholds to President Bashar Assad's forces.

Lebanon has been on edge since the central Syrian town of Yabroud fell to government troops on Sunday. Its rebel defenders started pouring into the Lebanese Sunni-dominated town of Arsal, which is surrounded by Shiite villages guarded by pro-Assad Hezbollah militants.

Syria's civil war already has already ignited sectarian violence between Lebanon's Sunnis and Shiites. The presence of rebels and Hezbollah in the same area risks a polarizing new flare-up.

On Monday, three rockets struck the predominantly Shiite towns of Labweh and Nabi Othman near Arsal, wounding at least one person and causing some damage, the army said.  The army said the rockets were fired from inside Syria.

Still, some angry Labweh residents claimed the rockets were fired from Arsal and closed off the main road between the two towns with sand barriers, guarded by about 20 armed Hezbollah fighters. They later closed other, smaller roads leading to Arsal, isolating the town from the rest of Lebanon.

Earlier in the day, Lebanese troops and commandos in desert camouflage patrolled the rugged border area on foot. On one patrol, near the northeastern village of Fakiha, they came across an abandoned SUV and fired a rocket-propelled grenade at it, turning the vehicle into a fireball and leaving a four-meter-wide (yard-wide) crater on the ground. They could not take the chance of it being a car bomb.

"We took the decision to blow it up immediately, without searching it," an officer told The Associated Press at the scene, speaking on condition of anonymity in line with military regulations.

In footage broadcast live on state television in Damascus, Syrian army officers raised the national flag in Yabroud's main square on Monday and covered a rebel flag with banners praising Assad's troops.

The fall of Yabroud, a smuggling hub for the rebels trying to overthrow Assad, was a major gain for Syrian government troops and their Hezbollah allies. It was also the Syrian opposition's last stronghold in the vital border area. The campaign consolidated the government's hold on the capital Damascus and the central Syrian city of Homs.

Yabroud's fall came after months of fighting in the mountainous Qalamoun region between Assad's forces and Hezbollah fighters on one side and the rebels, mostly Islamist militant groups, on the other.

The Hezbollah fighters have been instrumental to Assad's success on the battlefield, and support from the Iranian-backed group appears to have tipped the balance into the government's favor in Yabroud.

In Lebanon, Sunni militants have in the past weeks carried out several suicide bombings and car bomb attacks in Shiite dominated towns and suburbs of Beirut that are Hezbollah strongholds, claiming they were revenge for the group's role in the Syrian war.

On Monday, a Lebanese militant Sunni group claimed responsibility for a car bombing the previous night in Nabi Othman, a predominantly Shiite town in Lebanon's Bekaa Valley that also has a significant Christian community.

The Nusra Front in Lebanon said in a statement posted on its Twitter account that the attack, in which two people were killed and 14 were wounded, was in revenge for Hezbollah's support for Assad and "a quick response" for the fall of Yabroud into Syrian government hands.

Syria's 3-year-old conflict has devastated the country, killing more than 140,000 people and forcing millions from their homes.

The crisis started as largely peaceful protests against Assad's rule in March 2011. It turned into a civil war after opposition supporters took up arms to fight a brutal government crackdown on dissent. Over the past year, the conflict has taken increasing sectarian overtones, pitting predominantly Sunni Muslim rebels against Assad's government that is dominated by Alawites, a sect in Shiite Islam.

From the start, the rebels have been outgunned by the Syrian army, which has relied heavily on its air force to batter rebel-held regions. However, the rebels' resolve to overthrow Assad was significantly weakened after rival rebel groups, often backed by local tribal militias, turned on each other in battles over areas they had previously captured together from government forces.

More than 3,000 rebels have been killed in the infighting, and a spokesman for the Al Qaeda-linked Nusra Front in Syria's Qalamoun region blamed the fall of Yabroud on rebel-on-rebel clashes and rivalries.

"Yabroud did not fall. Yabroud was handed over to the (Syrian) regime and Hezbollah," said the spokesman, Abdullah Azzam al-Shami in comments posted on a militant website Monday.

He said Nusra front fighters in Yabroud were determined to hold the town but had to withdraw after rebels from other groups abandoned their positions in the surrounding hills, opening the way for Assad's troops to push in from the east.

The Syrian Nusra Front's relation to the much smaller Nusra Front in Lebanon is unclear.

Also Monday, Syria's state media reported that a mortar round struck the central Umayyad square of Damascus, killing one person and wounding two.

State TV also reported that a car bomb exploded in Homs Monday afternoon, killing six people and wounding more than 20.

Meanwhile, Human Rights Watch said the Syrian government "should immediately and unconditionally release the arbitrarily detained human rights defender" Mazen Darwish, who has been held since early 2012. It also called for the release of two of his colleagues.