The offensive is targeting insurgents from the Nusra Front group in the Juroud Arsal area on the outskirts of the Lebanese town of Arsal, and in areas of the western Qalamoun mountains in Syria.
The Lebanese Shia group Hezbollah and the Syrian regime forces launched an offensive on Friday to clear militants from an area of the Syrian-Lebanese frontier, a commander in the alliance fighting in support of regime leader Bashar al Assad said.
The operation is targeting insurgents from the Nusra Front group in the Juroud Arsal area on the outskirts of the Lebanese town of Arsal, and in areas of the western Qalamoun mountains in Syria, the commander said.
The Qalamun region lies on the Syrian side of the border across from Arsal. While much of it is now under regime control, pockets of rebel-held territory remain, particularly along the border.
The town of Arsal lies on the Lebanese side of the ambiguously demarcated border.
Around 45,000 Syrian refugees registered with the United Nations live in the town, but more are believed to be living in camps in the Jurud area, although their numbers have not been confirmed.
On Friday, a Lebanese security source said that many refugees have fled the area.
"The army is facilitating the passage of the displaced with supervision from UN delegates," the source said.
Lebanon deploys reinforcements
The Lebanese army has deployed reinforcements on the outskirts of the town. On Friday a Lebanese security source said that the army was in a defensive position.
"If our positions are not attacked we will not initiate (fire)," the source said.
The Nusra Front was Al Qaeda's official affiliate in the Syrian civil war until last year when it formally severed ties to Al Qaeda and renamed itself. The group now spearheads the Tahrir al Sham alliance.
Lebanese Prime Minister Saad al Hariri said on Tuesday that the Lebanese army, would carry out a carefully planned operation in the Juroud Arsal area, but there was no coordination between it and the Syrian regime forces.
Role of Hezbollah in Syria
In 2014, Arsal area was the scene of one of the most serious spillovers of the Syrian war into Lebanon, when militants briefly overran the town of Arsal.
The Iran-backed Shia group Hezbollah has provided the Syrian regime with crucial military support in the war, a role that has drawn heavy criticism from its Lebanese opponents including Hariri.
Hezbollah has supported the regime since 2012. In the beginning, it argued that its fighters were in Syria to protect Shia holy shrines and Lebanese villages on the border.
However, after the regime began to lose ground, Hezbollah began fighting on its behalf. Later, the militia's intervention extended to Sunni areas such as in Aleppo.