At least 50 rebels were killed in an ambush by Syrian government forces in a mountainous area near the Lebanese border on Saturday, activists and security sources said, Reuters reported.
Forces loyal to President Bashar al-Assad backed by allies from the Lebanese Shi'ite militant movement Hezbollah have largely pushed insurgents from towns and villages in the Qalamoun region bordering Lebanon since March, but some pockets of rebels have held out.
On Saturday, pro-government militias and Hezbollah fighters led an artillery ambush killing "dozens" of rebels, including from the al Qaeda-linked Nusra Front and the Islamic State, an al Qaeda offshoot, the British-based Syrian Observatory for Human Rights said.
The ambush took place around the town of al-Jobeh, around 10 km (6 miles) from the border.
The Observatory's director Rami Abdurrahman said it was not clear exactly how many rebels died in the fighting in the region, but the total was "at least 50".
Lebanese and Syrian security sources put the death toll much higher, saying around 170 rebels were killed, but this could not be independently confirmed. They said nine Syrian government fighters and two Hezbollah fighters had also died.
Tensions also flared across the border in Lebanon in the Sunni Muslim border town of Arsal, where tens of thousands of Syrian refugees have fled the fighting and violence from the war has often spilled over.
Lebanese authorities arrested a Nusra Front leader, Emad Jumaa, at a checkpoint around Arsal, security sources and state media said.
Following the arrest, "masked Islamist gunmen" appeared in the area, Lebanon's National News Agency said, although no clashes were immediately reported.
Syria's rebels are overwhelmingly Sunni, while Assad is from the Shi'ite-derived Alawite sect and has been supported by Shi'ite militias from Lebanon and Iraq. Over 170,000 people have died in the conflict, the Observatory says.