BEIRUT — 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.

The Lebanese Armed Forces pressed their offensive Sunday in a mountainous region bordering Syria, capturing new hills from IS. A bomb struck one of the advancing armored vehicles, killing the soldiers.

Lebanese military spokesman Col. Fadi Boueid said the blast that killed the three soldiers also critically wounded a fourth, He said troops were able to destroy a vehicle and a motorcycle driven by IS suicide attackers before they could hit their targets, and that 15 militants were killed on Sunday as a result of fighting and bombardment.

Boueid told reporters the army has so far captured two-thirds of the Lebanese land that was held by the extremists. He said on Sunday alone, troops captured 30 square kilometers (12 square miles).

Military experts are dismantling explosives left behind by IS fighters, and the army destroyed 12 militant positions in the border area, he said.

“There is no specific time on when the operation will end, but we will continue until we wipe out Daesh inside Lebanese territories,” Boueid said, using an Arabic acronym to refer to the group.

Another military 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.

The Syrian army and its ally, the Lebanese Hezbollah group, have launched a simultaneous operation 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 reported that Syrian warplanes carried out their most intense airstrikes Sunday since the operations against the extremists began.