Lebanon's powerful Hezbollah group said on Friday its fighters had begun an operation against militants on both sides of the country's border with Syria.
The operation has been anticipated for several weeks, and comes after Lebanese soldiers carrying out raids on Syrian refugee camps in the area were met with suicide bombings and a grenade attack.
That attack heightened tensions over the presence in Lebanon of more than a million refugees from the civil war across the border.
Hezbollah media outlets announced "the start of a military operation to purge Jurud Arsal and Qalamun of armed terrorists".
Jurud Arsal is a mountainous area around the Lebanese border town of Arsal, which lies directly across from the Syrian Qalamun region.
Hezbollah media said the operation had been launched on two axes -- from the Syrian town of Flita and from the southern part of Jurud Arsal, already under the group's control.
There was intermittent shelling throughout the day on the outskirts of Arsal, but the town itself remained relatively calm, a local source told AFP.
A Hezbollah statement provided to AFP confirmed that two of its fighters were killed in the clashes on Friday.
Another source from within the group, speaking on condition of anonymity, said the toll was as high as five.
Last week, Hezbollah chief Hassan Nasrallah, whose fighters have been battling inside Syria alongside government forces for years, warned that extremists in Jurud Arsal were "a threat to all, including the Syrian refugee camps".
"It is time to end this threat," he said.
On Tuesday, Prime Minister Saad Hariri said "the Lebanese army would conduct a planned-out operation in Jurud Arsal and the government gives it freedom (to do so)."
But there was little official comment on the Hezbollah offensive, with Lebanese President Michel Aoun saying he was "following the military developments" without elaborating.
Speaker of Parliament Nabih Berri discussed the Arsal clashes with Syrian ambassador to Lebanon Ali Abdulkarim.
"We discussed the consecutive victories in Arsal and in Syria," Abdulkarim said after Friday's the meeting, Syrian state news agency SANA reported.
There was no official comment from the Lebanese army on whether it was taking part, but it did say it had "authorised a group of women and children coming from a (refugee) camp close to the position of armed men to enter the town of Arsal".
The border between Lebanon and Syria is ambiguously demarcated in places around the town of Arsal, which is home to around 45,000 Syrian refugees registered with the United Nations.
Other refugees are believed to be living in camps in the Jurud area, directly on the border, cut off from the town by checkpoints, but their numbers have not been confirmed.
While much of Qalamun is now under Syrian government control, pockets of rebel-held territory remain, particularly along the border.
Ahead of the operation, the head of the International Committee of the Red Cross in Lebanon, Christophe Martin, told AFP that preparations had been made to receive wounded civilians from Jurud Arsal.
"We support two hospitals in Arsal and we have supplied them with more medicine and surgical material because they have limited resources," he said.
More than a million Syrian refugees have flooded into Lebanon since the conflict in their country erupted with anti-government protests in March 2011.
They live in apartments and homes but also informal camps across the country, and their presence has been largely tolerated despite testing the limited resources and ageing infrastructure of a country of just four million citizens.
But the security situation in Arsal has long been a concern, with Lebanese security forces fighting fierce battles with jihadists in the area in 2014.
When the jihadists withdrew, they kidnapped 30 soldiers and policemen, subsequently killing four of them while a fifth died of his wounds.
Sixteen were released in a prisoner swap in December 2015 but another nine are still being held.
Friday's operation comes at a time of rising frustrations over the refugee presence, with growing calls for a plan to repatriate the enormous population.
The Lebanese president warned this week against anti-refugee rhetoric in the wake of the June 30 attack on troops in Arsal.
After the raids, the Lebanese army arrested dozens of people, four of whom were subsequently declared to have died of pre-existing conditions while in custody.
Human rights groups have demanded an independent investigation into allegations that up to five Syrian detainees were tortured to death.