Hezbollah has seized ground from insurgents near the Lebanon-Syria border, the group said on Wednesday, widening its joint offensive with the Syrian army to try to clear the area of militant groups including al Qaeda's Syrian wing.

The Iran-backed Lebanese group said in a statement its fighters had seized three hilltops in the mountainous area east of the Lebanese town of Arsal, the target of an incursion last August by the al Qaeda-linked Nusra Front and Islamic State.

Hezbollah fighters were now advancing towards a strategically important peak, it said.

Hezbollah's support has been crucial to Syria's President Bashar al-Assad in the four-year-long Syrian conflict. Its operation in the Qalamoun mountains north of Damascus is part of Assad's effort to shore up his control over western Syria.

Assad has lost swathes of northwestern and eastern Syria in the last two months to an array of insurgent groups including both the Nusra Front and Islamic State, putting him under the toughest military pressure he has faced since the war began.