NORTHERN SYRIA -- Under the gate to Raqqa's old city, the bodies of five dead ISIS fighters rot in the desert heat. They were killed a week ago, American-backed militiamen told CBS News' Holly Williams, one of them still wearing a suicide belt.

The stench of death is everywhere in Raqqa, and so is fear. Getting to the front line must be done at breakneck speed, to avoid lurking ISIS snipers.

There's the thud of U.S. coalition airstrikes, but Williams saw only sporadic fighting because ISIS, with as few as 250 fighters remaining in the city, is pinned down.

The body of an ISIS fighter, with a suicide belt still attached CBS News

They're in what's left of the main hospital and other buildings close by, and they have human shields. In this broken city, it's thought there are still around 2,000 civilians.

America's allies on the ground are out for revenge -- and many have lost family members to ISIS.

And with them is an American volunteer, Mike Hogan from Phoenix, Arizona, who was a waiter before he went to Syria six months ago.

Mike Hogan, right, and Holly Williams CBS News

"Have you killed any ISIS fighters during the action that you've seen?" Williams asked him.

"No. Between you and me, I'm hoping to," he said. "I've had friends die here … I want to get even before I get out of here."

They've already retaken Clock Tower Circle, where ISIS acted out its perverted version of Islam. It used to be a landmark in a peaceful city, but ISIS turned it into a killing field.

Hasan Sherif CBS News

Hasan Sherif, a taxi driver who just escaped from the city, told Williams about the Yazidi women -- a religious minority -- captured by ISIS as sex slaves and auctioned off in the middle of the city.

"They'd shout '100 dollars for this one,'" he said "They did it to terrify us, to show they could do it to us if we didn't obey them."

There have been 75 U.S. coalition air strikes in the last two days alone, according to a senior U.S. official, preparing the ground for a final assault on Raqqa.