Syrian regime forces were Monday readying an assault on the last rebel-held areas in the Qalamoun mountains, strategically located on the Lebanese border, after overrunning a key opposition bastion.
The capture of the town of Yabrud on Sunday by Syrian troops and Hezbollah fighters came shortly after the conflict entered its fourth year and marked a significant setback for the rebels as it severs their supply lines from across the border.
It also raised fears of further spillover from the conflict into Lebanon, where Sunni extremists carried out a suicide car bomb attack late Sunday in a Hezbollah-dominated area that killed two members of the Shiite group, including a local official.
A security source in Damascus said the army would soon launch operations "in all areas where terrorists are to be found," using the regime term for rebels battling to overthrow President Bashar al-Assad.
"The aim of the army operation is to entirely secure the border and to close all corridors to Lebanon."
The fighting along the border has sparked a fresh flight of civilians into Lebanon, which is already hosting nearly a million refugees, according to the UN High Commissioner for Refugees.
"So far 150 families have crossed the border since Yabrud fell," said the UNHCR's Lisa Abu Khaled.
"NGOs there were on standby so they have handed out food, blankets, etc, and the UN plans to register the arrivals this week."
Speaking to AFP via the Internet from the Qalamoun mountains, activist Jawad al-Sayed said all civilians were evacuated from Yabrud before the town fell, either to areas nearby or to neighbouring Lebanon.
The road, he said, was dangerous, echoing reports from a day earlier that at least six people, including two children, were killed in air strikes as they fled for Lebanon.
"The situation of the civilians is very sad... So we have two options, either to go to Lebanon... or to stay here and resist," said Sayed.
The car bomb attack was claimed by Al-Nusra Front in Lebanon, which described it as a "quick response to the bravado... of the party of Iran (Hezbollah) for their rape of Yabrud".
On Monday four rockets launched from across the border with Syria hit eastern Lebanon, injuring one man, according to the Lebanese army.
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The Lebanese military was deployed in force on dirt roads and border crossings in the country's east, in a bid to avert "the infiltration of car bombs and armed men," said the official National News Agency.
Nineteen Syrian men and two Lebanese were arrested Monday after crossing the border illegally, said the military, adding that the men had in their possession an assault rifle, two pistols, cash and 30 mobile phones.
- Hezbollah commando raid -
The fall of Yabrud came after five months of Syrian army operations in the Qalamoun region, and more than 30 days of heavy aerial bombardment of the town.
A source close to Hezbollah in Lebanon's Bekaa Valley told AFP that the victory came after a Hezbollah commando raid on Yabrud that killed 13 rebel leaders, leaving their forces in disarray.
Among those killed, said the source, was Abu Azzam al-Kuwaiti, a key commander in Al-Nusra Front, Al-Qaeda's official Syrian affiliate.
Yabrud was once home to some 30,000 people, including a Christian minority, and had been a rebel bastion since early in the Syrian uprising, which began in March 2011 in the form of peaceful protests.
The town is a strategic prize because of its proximity to the highway and the Lebanese border, across which the mostly Sunni rebels have smuggled fighters and weapons.
Hezbollah's involvement in Syria has prompted bomb attacks by extremist groups against areas in Lebanon sympathetic to the Shiite movement, killing mostly civilians.
Syria's three-year conflict has claimed an estimated 146,000 lives and displaced millions of people.
In violence elsewhere in the country on Monday, a car bomb killed six people and wounded 20 in a district of Homs which is home to members of Assad's Shiite offshoot Alawite faith, according to the Syrian Observatory for Human Rights.
Further north in Aleppo a regime helicopter dropped an explosives-filled barrel on a rebel-held area and at least one man was killed in regime shelling, said the Observatory, a Britain-based group that relies on activists and other witnesses across Syria.