Three Lebanese soldiers were killed Sunday when a mine exploded under their vehicle in the east of the country along the border with Syria, the army said in a statement.
The soldiers were en route to participate in an operation that began Saturday to oust Islamic State group militants from an area further north on the border, an army source told AFP.
"An army vehicle was hit by a landmine on the Al-Najsa road roundabout in Jurud Arsal at noon today, killing three soldiers and injuring one soldier seriously," the army said in a statement.
It said the wounded soldier was evacuated to a hospital for treatment.
The incident comes a day after the army began an operation against IS in the Jurud Ras Baalbek and Jurud al-Qaa areas, further north along the Lebanon-Syria border.
The army said late Saturday its forces have so far captured around 30 square kilometres (11 miles square) of the 120 square kilometres IS is believed to hold in the area.
A spokesman said Saturday that 20 IS fighters had been killed in the clashes so far, and 10 Lebanese soldiers had been wounded.
The army estimates there are around 600 IS fighters in the areas it is battling to capture.
The army's assault comes after Lebanon's powerful Shiite militant group Hezbollah waged its own campaign against jihadists in Jurud Arsal.
That six-day offensive against Al-Qaeda's former affiliate ended with a ceasefire deal.
The agreement saw around 8,000 refugees and militants transported to a jihadist-held area of northwestern Syria in return for the release of five captured Hezbollah fighters.
Security along Lebanon's eastern border with Syria has long been a concern, and in 2014 jihadists invaded the border town of Arsal and kidnapped 30 Lebanese soldiers and police.
Four were executed by their captors while a fifth died of his wounds.
Sixteen were released in a prisoner swap in December 2015, but another nine solders are believed to remain in IS hands.