There are between 250 and 400 fighters from the Islamic State and Fatah al-Sham, as well as some 800 of Saraya Ahl al-Sham - a group loyal to the Free Syrian Army, who are entrenched on the outskirts of Arsal and other areas along the Lebanese-Syrian border.
The shelling is concentrated on the outskirts of Flita, an area in Syria but close to the Lebanese-Syrian border and Wadi al-Kheil on Arsal's outskirts, where the Fatah al-Sham Front has positions.
A still image taken on July 21, 2017 from an undated footage by Hezbollah Military handout shows the area at the Syrian-Lebanese border, taken by a drone at an unidentified location at the Syrian-Lebanese frontier.
Hezbollah Lebanese resistance fighters, backed by Syrian warplanes and artillery, were targeting Nusra Front gatherings in Arsal outskirts and Qalamoun.
The army is facilitating the passage of Syrian refugees fleeing the area, with United Nations supervision, the security source said.
Meanwhile, the Lebanese army reinforced its outposts in anticipation of any responses from the militants against army posts in Arsal or the northern Bekaa region.
Hariri, however, said "there is no coordination between the Lebanese and Syrian armies".
On Tuesday, Lebanese Prime Minister Saad Hariri said the country's military was preparing an operation to secure a lawless section of the border with Syria, and Hezbollah leader Sheikh Hassan Nasrallah suggested in a speech last week that a joint operation was in the works with the Lebanese and Syrian militaries to expel insurgents from the border area.
There was no official indication on Friday on whether Lebanese troops were involved in the fighting.
The militant-held areas are surrounded from all sides, leaving them with no place to withdraw to.
Almost 3,000 militants, two-third of which are from Daesh and al-Nusra Takfiri groups, have been using the barren area in the mountains between Syria and Lebanon as a base to organize attacks inside Syria.
Hezbollah says its counterterrorism mission in Syria is aimed at preventing the spillover of the crisis into Lebanon.
The militants managed to kidnap around 30 Lebanese servicemen in the incident and nine are still in Islamic State captivity.
Friday's fighting between the ultraconservative Ahrar al-Sham and the al-Qaida-linked Levant Liberation Committee focused on the Bab al-Hawa crossing. The group now spearheads the Tahrir al-Sham alliance.
The group said three days of fighting between the two rival groups left 85 people dead, including 68 fighters from both sides.
Gold Cup soccer matches to snarl rush hour traffic in Glendale
I don't know an bad lot (and there are legions of people who will be quick to agree with that) but I do know when I'm entertained. The Mexican team was without many top players who had played in the recently completed Confederations Cup in Russian Federation .