By Laila Bassam and Tom Perry

BEIRUT (Reuters) – Lebanon’s Hezbollah and the Syrian army launched an offensive to drive insurgents from their last foothold on the Syrian-Lebanese border on Friday, a pro-Damascus military commander said.

The operation targeted insurgents from the Nusra Front group in the mountainous outskirts of the Lebanese town of Arsal and areas near the Syrian town of Fleita, the commander said.

Media run by Hezbollah reported significant gains by its side in the early stage of the operation.

A Lebanese security source said refugees living in the area were fleeing toward Arsal and the Lebanese army was facilitating their passage with U.N. supervision.

U.N. refugee agency (UNHCR) spokeswoman Lisa Abou Khaled said only a small number of people had fled to Arsal town so far.

“UNHCR has only received confirmation … that two Syrian families have arrived in the town of Arsal from the outskirts,” she said.

Several thousand Syrian refugees occupy camps east of the town in an area known as Juroud Arsal, a barren mountainous zone between Syria and Lebanon that has served as a base for Islamic State militants, jihadists and other rebels fighting in Syria’s six year civil war.

Hezbollah’s al-Manar TV said Nusra militants were under attack in Juroud Arsal and in areas near the Syrian town of Fleita. A military news outlet run by Hezbollah reported Syrian army air strikes on Nusra positions near Fleita.

Al-Manar broadcast footage showing an artillery gun being fired from the back of a truck flying the Hezbollah flag. Plumes of smoke were shown rising from the hills.

Hezbollah, a Shi’ite group backed by Iran, has played a critical part in previous campaigns against insurgents along the border, part of the much wider role it has played supporting President Bashar al-Assad in the Syrian war.

The Lebanese army is not taking part in the operation, the commander in the pro-Damascus military alliance and the Lebanese security source said. The Lebanese source said the army had assumed a defensive position, was monitoring militant movements, and would fire if it came under attack.

The Lebanese National News Agency later reported that the army had fired on a group of militants trying to flee the fighting toward Arsal town.

ARMY REINFORCEMENTS

The Lebanese army, a recipient of U.S. and British military support, deployed reinforcements on the outskirts of Arsal in anticipation of the operation this week to prevent militants from crossing into Lebanon.

Hezbollah’s role in the Syrian war has been a major point of contention in Lebanon, facing criticism from opponents including Prime Minister Saad al-Hariri. Hariri’s Future Movement on Thursday said the anticipated Arsal battle was part of “the services” offered by Hezbollah to “the Syrian regime”.

Hariri said on Tuesday 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 army.

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 Islamist alliance.

In 2014, Arsal was the scene of one of the most serious spillovers of the Syrian war into Lebanon, when jihadists briefly overran the town.

Negotiations failed to secure the militants’ withdrawal from the Juroud Arsal area to other rebel-held parts of Syria.

Earlier this month, Hezbollah leader Sayyed Hassan Nasrallah said time was running out for Syrian militants along the border near Arsal to reach deals with Syrian authorities, saying it was “high time to end the threat of militant groups in Arsal”.

(Reporting by Laila Bassam/Tom Perry; Writing by Tom Perry; Editing by Jon Boyle and Hugh Lawson)