Lebanese army spokesman Ali Qanso, left, gestures during a news conference at the Defense Ministry in Yarze. (Anwar Amro/AFP/Getty Images)

The U.S.-backed Lebanese army launched a long-awaited offensive on Saturday against Islamic State militants holed up in a remote stretch of northeastern Lebanon, just as a separate offensive by the Hezbollah militia and the Syrian army got underway right across the border in Syria.

The offensive is the biggest military operation launched by the Lebanese army since the Syrian rebels and extremists began infiltrating parts of northeastern Lebanon after the outbreak of war in Syria in 2011, and, if successful, will enable Lebanon to reassert control over all of its borders.

The battle is fraught with sensitivities, however, because of the dueling roles played by the U.S.-backed Lebanese force and Iran-backed Hezbollah, which operate alongside one another as both allies and rivals in Lebanon’s complicated political landscape. Hezbollah is a partner in Lebanon’s coalition government, from which the Lebanese army takes its orders.

But their sponsors put them at opposite ends of a wider spectrum of geopolitical rivalries playing out in Lebanon and across the Middle East — between the United States and Iran.

This is a fight the historically weak and divided Lebanese army cannot afford to lose, said Aram Nerguizian in an analysis for the Washington-based Center for Strategic and International Studies late last month.

“Failure, or the risk of it, would only bolster Hezbollah’s argument that it and Iran are indispensable to Lebanon’s stability,” he said.

The Lebanese army insisted that there was no coordination with the Hezbollah and Syrian forces, whose operation was confined to the Syrian side of the border. The Lebanon-based Hezbollah militia has been fighting for years alongside the Syrian army in support of Syrian President Bashar al-Assad.

Lebanese army troops launched their assault at dawn near the towns of Ras Baalbek and Qaa, located in rugged, mountainous terrain that has been under Islamic State control since 2014, said Brig. Gen. Ali Qanso at a news conference at the Defense Ministry, north of Beirut.

“There is no coordination, neither with Hezbollah nor with the Syrian army,” he said. The Lebanese army launched its operation first, he said, pounding militant positions with rocket and artillery fire starting at 5 a.m.

Hezbollah issued a separate statement saying that it began its offensive early Saturday, with no mention of the Lebanese army operation.

The issue of coordination between the Lebanese army and Hezbollah is sensitive because of the extensive military aid Lebanon’s army receives from the United States, which considers Hezbollah to be a terrorist organization.

More than $1.5 billion in U.S. military aid has come to Lebanon over the past decade, including supplies of tanks, armored personnel carriers, surveillance drones, attack aircraft and helicopters that account for 80 percent of the military equipment in use by the Lebanese army, according to a fact sheet provided by the U.S. departments of Defense and State. U.S. military trainers have trained 32,030 Lebanese troops, nearly half of the strength of the army, it said.

The latest delivery, eight Bradley fighting vehicles, came just days before the offensive was launched, and top U.S. military officials, including Central Command’s Gen. Joseph Votel, endorsed the army’s battle against militants with a visit to the area in June.

“The Lebanese Army remains one of our most capable and efficient partners,” he said, according to comments quoted by a Lebanese army statement. “We are proud of our support for it as the sole defender of Lebanon.”

The aid has gone a long way toward turning Lebanon’s historically weak and politically divided army into an effective fighting force, but the battle ahead is nonetheless a challenge. The Islamic State has controlled the area for three years and knows the forbidding terrain well, said Qanso, the Lebanese army general.

Though there are only about 600 militants in the 46-square-mile area, they are hiding in networks of caves dug into the mountains and are expected to use suicide attacks to defend their territory, Qanso said.

Haidamous reported from Washington.