BEIRUT (AP) — Lebanon's U.S.-backed army on Saturday launched its biggest military operation yet against Islamic State militants, who gained a foothold along the border with neighboring Syria in 2014.

The long-awaited campaign aims to defeat the militants in their last enclave and put an end to a terror threat that has loomed over the country for years.

The Lebanese Hezbollah group and the Syrian army announced a simultaneous offensive to clear IS militants from the Syrian side of the border, in the western Qalamoun mountain range. Hezbollah has been fighting alongside President Bashar Assad's forces in Syria since 2013.

Lebanese President Michel Aoun called field commanders from the Defense Ministry where he was monitoring operations early to tell them, "You will not disappoint us," and "Our hearts and minds are with you," according to video broadcast on TV channels across the country.

Operations commenced before dawn, with the military striking IS positions in the eastern border areas with Syria, Brig. Gen. Ali Qanso said in a press conference at the Defense Ministry. He warned of a difficult battle ahead. The barren hills in eastern Lebanon will leave infantry exposed to IS snipers, and the militants are expected to mine the area on a vast scale.

A group of IS militants, including a self-styled "emir," or local commander, surrendered to the advancing Hezbollah and Syrian forces by midday, according to Hezbollah's Central Military Media outlet, which is run jointly with the Syrian military.

Lebanese authorities insist they are not coordinating with Syrian President Bashar Assad's government, and Qanso said Saturday the army was not even coordinating with Hezbollah. That would spare Washington the potential embarrassment of appearing to be allied with a group it classifies as a terror organization. The U.S. is a key patron of the Lebanese army, and top brass from the Pentagon visited Lebanon in the run-up to the operation.

Nevertheless, Hezbollah ministers hold key portfolios in the Lebanese government, and the country is now accustomed to seeing the militant group run operations in parallel with the army.

The presence of extremists in the border area has brought suffering to neighboring towns and villages, from shelling to kidnappings for ransom. Car bombs made in the area and sent to other parts of the country, including the Lebanese capital, Beirut, have killed scores of people.

The army has accumulated steady successes against the militants in the past year, slowly clawing back territory, including strategic hills retaken in the past week.

Lebanon has been spared the wars and chaos that engulfed several countries in the region since the so-called Arab Spring uprisings of 2011. But it has seen sectarian infighting and random car bombings, particularly in 2014, when militants linked to al-Qaida and IS overran the border region, kidnapping Lebanese soldiers.

Qanso said the army had no new information about the whereabouts of the soldiers kidnapped by IS militants.

Lebanese politicians say IS controls an area of about 300 square kilometers (115 square miles) between Syria and Lebanon, around half of which is in Lebanon.

About 600 jihadists, including snipers, are holed up in the Lebanese portion of the enclave, according to Qanso. They are armed with guided rockets, anti-armor rockets, mortars, and other heavy, medium, and light weaponry.

The area stretches from the Lebanese town of Arsal, and Christian villages of Ras Baalbek and Qaa, to the outskirts of Syria's Qalamoun region and parts of the western Syrian town of Qusair, which Hezbollah captured in 2013.