BEIRUT — With all eyes on the Islamic State group’s onslaught in Iraq and Syria, a less conspicuous but potentially just as explosive front line with the extremists is emerging in Lebanon, where Lebanese soldiers and Shiite Hezbollah guerrillas are increasingly pulled into deadly fighting with the Sunni militants along the country’s border with Syria.

The U.S. has been speeding up delivery of small ammunition to shore up Lebanon’s army, but recent cross-border attacks and beheading of Lebanese soldiers by Islamic State fighters has sent shock waves across this Mediterranean country, eliciting fear of a potential slide into the kind of militant, sectarian violence afflicting both Syria and Iraq, and increasingly prompting minorities to take up arms.

The crisis was slow in coming.

For long, Lebanon managed to miraculously avoid the all-out chaos gripping neighboring countries — despite sporadic street clashes and car bombings, and despite being awash with weapons and taking in an endless stream of refugees from Syria who now constitute a staggering one third of its population of 4.5 million people.

Unlike in Syria or Iraq, the al-Qaida-breakaway Islamic State group does not hold territory in Lebanon. But along with Syria’s al-Qaida affiliate, the Nusra Front, it has established footholds in remote mountains along Lebanon’s remote eastern border, from where it launches almost daily incursions further afield.

Jihadi recruitment in impoverished Sunni areas of northern Lebanon is on the rise, and black Islamic State group flags fly freely in some areas, reflecting pockets of growing support for the radical group.

“Lebanon is in the eye of the storm,” said Fadia Kiwan, a political-science professor at Beirut’s St. Joseph University.

The Lebanese are bitterly divided over Syria’s civil war. Hezbollah fighters have gone to join Syrian President Bashar Assad’s forces in their battle against Sunni rebels, drawing anger at home from Lebanon’s Sunnis and stoking Sunni-Shiite tensions.

This in turn led to tit-for-tat suicide bombings and several rounds of street clashes in Lebanon in the past year.

The Islamic State group threat first came to Lebanon in August, two months after the group’s summer blitz in which it seized large swaths of territory in Iraq and Syria.

In a surprise attack, Islamic State group and Nusra Front militants crossed over from Syria and overran the predominantly Sunni Lebanese border town of Arsal, hitting Lebanese army positions and killing nearly 20 soldiers.

After weeklong clashes, the militants pulled back to mountain caves near Syria’s border, taking more than 20 Lebanese soldiers and policemen with them.

Islamic State fighters have since beheaded two Lebanese soldiers. Nusra Front militants have shot dead a third.

In return for remaining hostages, they have issued various demands, including the withdrawal of Hezbollah troops from Syria, and the release of Islamists from Lebanese prisons.