The U.S. military says it was forced to hold its fire and watch helplessly as defeated Islamic State fighters fled the Syrian city of Manbij in convoys packed with civilian hostages, being used as human shields.

As many as several hundred vanquished Islamic State fighters escaped Friday as the U.S. used surveillance drones to try to determine if the vehicles could be bombed without the loss of innocent lives.

"Every vehicle had civilians that we could identify with our systems," said U.S. military spokesman Col. Chris Garver at a Pentagon briefing Tuesday.

"That was a decision by the [Syrian Democratic Forces] commanders on the ground, by the [Syrian Arab Coalition] commanders on the ground, was to let the convoy continue since they couldn't engage it. They let that convoy continue," Garver said.

Garver would not say where the Islamic State forces, which he estimated at as many as several hundred, went, except that it was to the north of Manbij, and that they didn't all go together or stay together.

"The coalition continues to track those forces," Garver said. "As it's an ongoing operation, we won't discuss the specifics."

So far none of the civilians who accompanied the fleeing Islamic State fighters has returned to Manbij, and Garver conceded some might be family members who were not hostages.

"We anticipate it was probably a mix of both," Garver said. "There were some that were being held unwillingly and some that were traveling willingly with them. We don't know that, which is why, you know, we had to treat them all as noncombatants. We didn't shoot. We kept watching."