Lebanese troops on Saturday launched an offensive against the Islamic State group on the country's eastern border with Syria, seeking to drive the jihadists from a long-time stronghold.
The operation came as IS faces multiple military attacks on territory it controls in both Syria and neighbouring Iraq, where it lost the city of Mosul in July.
Militants have long been active in mountainous eastern Lebanon near the border with Syria, where a bloody civil war has raged since 2011.
In 2014, jihadists invaded the border town of Arsal, capturing 30 Lebanese soldiers and police. Security forces in the region have since come under regular attack.
"In the name of Lebanon, in the name of kidnapped Lebanese soldiers, in the name of martyrs of the army, I announce that operation 'Dawn of Jurud' has started," army chief General Joseph Aoun said Saturday.
He was referring to two mountainous border areas -- Jurud Ras Baalbek and Jurud al-Qaa -- where IS has been active.
"The army is confronting the Daesh terrorists to chase them out and recover territory," army spokesman General Ali Qanso said, using an Arabic acronym for IS.
He said the campaign was "one of the most difficult battles waged by the Lebanese army," but added: "We have no fear of Daesh."
Kanso said the army believed there were around 600 IS fighters in the two areas, controlling some 120 square kilometres (46 square miles) of territory.
"For the first time, the army has made such use of its air capabilities," he said, to target jihadist hidden in a region full of mountains and cave hideouts.
The army's operation comes after Lebanon's powerful Shiite militant group Hezbollah launched its own campaign against jihadists in another border area south of where the military is now operating.
The group's six-day offensive against IS and Al-Qaeda's former affiliate in the Jurud Arsal area ended with a ceasefire.
The agreement saw around 8,000 refugees and jihadists transported to a jihadist-held area of northwestern Syria in return for the release of five captured Hezbollah fighters.
The evacuations were completed on Monday.
Hezbollah is deeply embroiled in the civil war that has raged in neighbouring Syria since 2011, fighting in support of President Bashar al-Assad's regime.
The group said Saturday it had launched a simultaneous operation against IS from the Syrian side of the border, but Lebanon's army spokesman denied there was any coordination.
Nine Lebanese soldiers captured in Arsal in 2014 are believed to remain in the hands of IS jihadists.
Four were executed by their captors while a fifth died of his wounds. Sixteen were released in a prisoner swap in December 2015.
Hezbollah chief Hassan Nasrallah vowed earlier this month to clear the whole border area of jihadists, saying it was in the interests of both Lebanon and Syria.
His powerful group is the one Lebanese party that did not turn over its weapons after the 1975-1990 civil war, and its arsenal remains a source of division in Lebanon.
The last major battle fought by Lebanon's army was the 2007 fight against jihadists in the Palestinian Nahr al-Bared refugee camp.
The three-month battle left more than 400 dead, including 168 soldiers and 220 jihadists.
Lebanon, a country of some four million people, hosts more than a million Syrian refugees, whose presence has caused tensions over limited resources.
Syria's conflict has killed more than 330,000 people and displaced millions since it erupted in March 2011 with anti-government protests.