An ongoing battle between the Lebanese army and radical Islamist militants along Lebanon's Syrian border is sparking fears that Lebanon might be pulled into the Syrian civil war.

Already, extremists from the militant group the Islamic State (formerly known as ISIL or ISIS) have taken over huge swaths of Syria and Iraq. If the rebels continue their assault on Lebanon, it could mark a new front in the militants' quest to form an Islamic caliphate across much of the Middle East.

A serious assault on Lebanon would also be alarming because there are already tensions between Sunni and Shia Muslims in Lebanon as well as elsewhere in the Middle East, and an attack by Sunni extremists could inflame that strife.

“That’s exactly what the [extremists] want — they want to pit Shia and Sunni against each other," Ariane Tabatabai, a Middle East security expert at the Belfer Center at Harvard University, told Mashable.

Gunmen drive away with about a dozen men, two in camouflage police uniforms, in Arsal, a Sunni Muslim town near the Syrian border in eastern Lebanon, on Aug. 2. Image: AP Photo/Associated Press

To Lebanon, a state riven by religious and sectarian differences and jealousies, the militants could pose an existential threat, even if they're relatively small in number.

“They can create all kinds of problems in terms of raising sectarian tensions," said Anthony Cordesman, a Middle East security expert at the Center for Strategic and International Studies.

The battle began last week in the border town of Arsal after Lebanese soldiers arrested Imad Ahmad Jomaa, a rebel commander in Syria who used to be a part of the al-Qaeda-affiliated Al Nusra Front before apparently swearing allegiance to the Islamic State.

His followers retaliated with an attack on Lebanese forces that ended up with the militants in control of Arsal. At least 20 militants and 17 soldiers were killed during fighting, according to The Associated Press. An additional 22 Lebanese soldiers are missing and may have been captured, according to The New York Times.

In recent months, the Islamic State has surprised even long-term observers with their swift and extremely brutal military campaign. Some observers, however, said that, with this latest assault, the group may be overextending itself.

Lebanese army reinforcements arrive to the outskirts of Arsal, a predominantly Sunni Muslim town near the Syrian border in eastern Lebanon, on Aug. 4. Image: Bilal Hussein/Associated Press

"The sheer ambition of their agenda gives them a tendency to overreach," said Faysal Itani, a fellow at The Atlantic Council's Rafik Hariri Center for the Middle East. "Then again, few people expected them to take Mosul [in Iraq] so quickly and to successfully hold it. They’ve proven to be cunning strategists."

Though the commander Jomaa and his followers could be seen on a YouTube video, pledging a new allegiance to the Islamic State, it is unclear whether the militants now count them among their own and — most importantly — if they are directing the attack on Lebanon.

“There are a lot of small cells, individual tribal groupings, jihadist factions," Cordesman said. "When you see somebody emerge as a really major source of power or money, you may choose to affiliate yourself.”