A mine blast killed three Lebanese soldiers on Sunday as they were heading to take part in an offensive against ISIL along the border with Syria, the army said.
Lebanon's army began its operation in the Ras Baalbek and Qaa region early on Saturday, and has already recaptured two-thirds of the territory in the area, the military said.
The retaken area covers about 80 square kilometres once controlled by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, the army said.
But an army source warned on Sunday that removing ISIL from the rest of the territory would be tough.
"The toughest battle will be to retake the remaining 40 percent because it is there that Daesh fighters are gathered. Nevertheless, the army is ready," said the source, using an Arabic acronym for ISIL.
The soldiers killed on Sunday were on their way to join the offensive to remove ISIL, the military said in a statement.
"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," it read.
The statement said the wounded soldier was evacuated to hospital.
The battle against ISIL is being fought further north from Jurud Arsal, along the border with Syria.
The army also said soldiers blew up an explosives-packed car and a booby-trapped motorcycle carrying would-be suicide bombers in Ras Baalbek, preventing an attack on the troops.
Simultaneous Hezbollah offensive
On Sunday, Lebanon's National News Agency reported soldiers used heavy artillery and rockets against ISIL positions in Jurud Ras Baalbek.
At least 20 ISIL fighters have been killed in clashes since the army unleashed its operation on Saturday, and 10 soldiers wounded, military spokesman Brigadier General Nazih Jreij said.
He said on the first day of the offensive that the army had captured around 30sq km of territory.
The army says around 600 ISIL fighters were deployed in the Jurud Ras Baalbek and Jurud al-Qaa areas and controlled around 120sq km of territory before the latest advances.
The operation ended with a ceasefire that saw around 8,000 refugees and fighters transported to a rebel-held area of northwestern Syria in return for the release of five captured Hezbollah fighters.
On Saturday, Hezbollah said it launched an operation against ISIL from the Syrian side of the border, where its fighters are battling alongside President Bashar al-Assad's army against rebels.
The Lebanese army denied any coordination with Hezbollah or the Syrian army.
Hezbollah's War Media outlet said its fighters and Syrian troops had "managed to liberate 87 square kilometres of the total area controlled by the Daesh organisation ... in western Qalamun region" of Syria on Saturday alone.
On Sunday, the SITE Intelligence Group reported an ISIL claim that the group had killed "nearly 100 Lebanese soldiers and Hezbollah fighters amidst clashes in western Qalamun".
Security along Lebanon's eastern border with Syria has long been a concern, and in 2014 ISIL and the al-Nusra Front group invaded the town of Arsal and kidnapped 30 Lebanese soldiers and policemen.
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 soldiers are believed to remain in ISIL hands, their fate unknown.
The army offensive against ISIL also comes after the group claimed several international attacks, including twin car-ramming incidents in Spain that killed 14 people.
Lebanese soldiers raised the Spanish flag on a hilltop captured from ISIL on Saturday in a tribute to the victims of those attacks, the army said.