Ash Sha'Fah, Syria — ISIS has claimed responsibility for a suicide bombing outside a restaurant in northern Syria, with four Americans among at least 16 people killed. The terror group once controlled vast territory in Syria and Iraq, but has been pushed into a few remaining pockets. But in northern Syria, U.S. and Kurdish forces are battling an estimated 2,000 ISIS fighters.

On the way to the front line, there's not much left of the villages that once stood. The ISIS fighters who used to hold them only days ago remain a lethal threat, launching counterattacks in areas thought to be secure and lacing the only road in and out with explosives.

It's an ISIS tactic to plant roadside bombs in the overnight or early morning. As CBS News got closer to the fighting, tractors and truckloads of residents were seen fleeing.

In Ash Sha'Fah, America's Kurdish-led allies launched a barrage of mortars, locked in a fight to the end against the remaining pockets of ISIS in Syria. But they're not the only ones. Americans are fighting too.

Commander Simko Shkak said the U.S. forces are playing a big role. They give them coordinates for ISIS locations, and their air force and artillery do all they can, he explained.

The American-led onslaught has been ferocious. The coalition reported 575 airstrikes on 1,100 targets during the first two weeks of this month alone. But while these ground forces are making progress, CBS News was told ISIS is still putting up a strong resistance.

Leaving the area, it was easy to see the destruction caused by some of the fiercest fighting the region has seen since the battle against ISIS began. For all the talk of withdrawing American troops, CBS News saw a convoy of U.S.-led coalition forces head straight for the front line.

One commander said ISIS is now fighting for their very existence in a fight that is far from over.