Lebanese soldiers renewed fire on ISIS group positions on the country's eastern border with Syria on Sunday after capturing a third of the territory held by militants in the area.
Lebanon's army began its operation in the Jurud Ras Baalbek and Jurud al-Qaa region early on Saturday, and in the first day captured around 30 square kilometers of territory, a military spokesman said.
"That is around a third of the area controlled by the terrorists," Brigadier General Nazih Jreij said late Saturday.
Jreij said 20 ISIS fighters had been killed in the clashes, and 10 Lebanese soldiers wounded.
Lebanon's battle against ISIS comes as the militant group faces assaults in both neighboring Syria and Iraq, where the government early Sunday announced an offensive on the group's bastion of Tal Afar.
The attack also comes after ISIS 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 ISIS on Saturday in a tribute to the victims of those attacks, the army said.
Lebanon's military estimated around 600 ISIS fighters were present in the Jurud Ras Baalbek and Jurud al-Qaa areas, controlling around 120 square kilometres of territory before Saturday's advances.
On Sunday, Lebanon's National News Agency reported soldiers were firing heavy artillery and rockets at ISIS positions in Jurud Ras Baalbek.
The army's assault comes after Lebanon's powerful Shiite militants group Hezbollah waged its own campaign against militants operating in another border area south of the current battle.
The group's six-day offensive against ISIS and Al-Qaeda's former affiliate in the Jurud Arsal area ended with a ceasefire.
The agreement saw around 8,000 refugees and militants transported to a militant-held area of northwestern Syria in return for the release of five captured Hezbollah fighters.
Hezbollah said Saturday it had launched a simultaneous operation against ISIS from the Syrian side of the border, where the group's fighters are battling alongside President Bashar al-Assad's army against rebels.
Lebanon's army denied any coordination with Hezbollah or the Syrian army.
Hezbollah's War Media outlet said its fighters and the Syrian army had on Saturday "managed to liberate 87 square kilometres of the total area controlled by the Daesh (ISIS) organization... in western Qalamun region" of Syria.
Security along Lebanon's eastern border with Syria has long been a concern, and in 2014 militants invaded the 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 ISIS hands.
It is not clear if they are alive or not.