Photo: The Canadian Press

Brig. Gen. Ali Qanso

Lebanon suffered its first casualties from a massive operation to drive the Islamic State group from an area along the Syrian border on Sunday when a roadside bomb killed three soldiers.

The deaths came a day after the U.S.-backed army launched its biggest military operation yet against IS, which gained a foothold along the tiny Mediterranean country's border with Syria in 2014.

On Sunday, the Lebanese Armed Forces pressed their offensive in a mountainous region bordering Syria, capturing new hills from IS. A bomb struck one of the advancing armoured vehicles, killing the three soldiers, a military official said.

The official said troops captured several hills on the outskirts of the Lebanese village of Ras Baalbek, adding that army commander Gen. Joseph Aoun visited areas regained from the extremists.

The official spoke on condition of anonymity in line with regulations.

Simultaneously, the Syrian army and its ally, the Lebanese Hezbollah group, are pushing to clear IS militants from the Syrian side of the border, in the western Qalamoun mountain range. Hezbollah has been fighting in Syria alongside President Bashar Assad's forces since 2013.

Hezbollah's Al-Manar TV meanwhile reported that Syrian warplanes carried out their most intense airstrikes Sunday since the operations against the extremists began.