Officers loyal to Syria's President Bashar al-Assad talk to the media in Yabroud on Sunday in this handout photograph released by Syria's national news agency SANA. (Sana/Reuters)

Lebanon was struck by a retaliatory car bombing on Sunday, just hours after its Hezbollah militants helped Syrian army troops seize a key town across the border, highlighting the increasing security risk to Syria’s smaller neighbor.

Syrian troops, backed by fighters from the Lebanese Shiite movement Hezbollah, faced little resistance as they swept into the center of the town of Yabroud, just a few miles over the Lebanese border, leading rebels to trade accusations that some factions had negotiated a withdrawal.

The Syrian army and Hezbollah have been tightening their noose on the Yaboud for months, part of a wider offensive to secure the rugged border area of Qalamoun, cutting rebel supply lines into Syria.

A little over 24 hours after pro-government forces first entered the town on Friday night, the official Syrian Arab News Agency, said that “terrorist groups” in the town had been “devastated” and that the area was being combed for explosives. Some clashes were still reported in the town as militants from the al-Qaeda linked Jabhat al-Nusra fought on.

The fall of the area’s main town is likely to strike a blow to rebel morale as, after three grinding years of conflict, President Bashar al-Assad steadily makes gains.

The estimated death toll from the violence in Syria is more than 146,000 as the conflict enters its fourth year. The Post's Liz Sly explains the impact on the region and what we can expect moving forward. (The Washington Post)

Col. Said Swaid, a rebel commander, said he left Yabroud at dawn after several rebel battalions “suddenly pulled out causing panic and fear.” He said a deal had been struck with Syrian government forces.

“I saw the betrayal with my own eyes,” he said. “Rebels walking with the regime army.”

Hezbollah has claimed the operation to take Yabroud, which it says is the source of explosive-packed vehicles that have exploded in Beirut, will help secure Lebanon.

However, within hours of Yabroud’s fall on Sunday, the Shiite town of Nabi Othman in Lebanon’s Bekaa Valley was struck by a suicide bombing.

At least four people died, according to al-Manar, Hezbollah’s television channel.

Earlier in the day, al-Manar had broadcast celebratory scenes from what it said was Yabroud’s town center Sunday. Men in fatigues raised a Syrian flag on a pole in the middle of the street, while another held aloft a portrait of Assad.

Celebratory gunfire and fireworks could be heard from the southern suburbs of Beirut, a base of support for Hezbollah, where regular car bombs have struck in retaliation for the movement’s backing of Assad.

The mood darkened with Sunday night’s bombing, the eighth suicide attack in Lebanon this year, driving home the ability of extremist groups to continue their activities. Jabhat al-Nusra claimed responsibility, saying the attack was in response to Hezbollah’s activities in Yabroud.

It disputed a claim of responsibility claim from a group calling itself the Ahrar al-Sunna of Baalbek.

Swaid, the rebel commander, said only Jabhat al-Nusra had stayed to fight in Yabroud on Sunday. In an unverified statement circulated online, Abdullah Azzam al-Shami, a spokesman for the group in Qalamoun, said Yabroud had been “delivered” to the Syrian government and Hezbollah after rebel leaders decided to withdraw.

Islam Alloush, a spokesman for the Islamic Front, an amalgamation of seven Islamist rebel groups, also said his fighters were also still engaged in clashes in Yabroud on Sunday, repelling attempts by pro-government forces to enter. He said four Hezbollah fighters had been killed.

Others rebels fled to the nearby towns of Rankous and Flita, or on to Lebanon. Fifty casualties from Yabroud were brought over the border to the Lebanese town of Arsal for treatment, said Kassem Zain, who runs a field hospital there. Eight people died, he said.

For most of the civil war, Yabroud had been largely sheltered from the fighting that has engulfed Syria, its population swelling with the internally displaced. The offensive has sent a new wave of refugees fleeing into Lebanon.