WASHINGTON D.C. -- A couple hundred vehicles of Islamic State of Iraq and Syria (ISIS) fighters were allowed to leave the northern Syrian city of Manbij as U.S.-backed forces seized the town in recent days because the militants had civilians with them, according to a U.S. military official.

The official said Tuesday that some of the ISIS fighters may have already made their way into Turkey, but many are still in Syria. The official was not authorized to discuss the matter publicly so spoke on condition of anonymity.

Col. Chris Garver, a spokesman for the U.S.-led coalition fighting ISIS, told Pentagon reporters that the decision to let the convoy leave the city was made by commanders of the Syrian Democratic Forces. He said there were civilians in each of the vehicles, and the military wanted to avoid casualties. He added that he doesn't know how many of the civilians may have been in the cars voluntarily, but some were likely hostages.

It's not clear if the militants left under a pre-arranged agreement between the SDF and the ISIS fighters. During the offensive, the SDF had offered fighters a safe route to leave the town but they refused.

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ISIS has repeatedly used civilians as human shields, including in recent battles in Iraq.

"They kept throwing civilians to basically walk into the line of fire, trying to get them shot to use that potentially as propaganda, we think," said Garver.

Garver said the coalition has been tracking and watching the vehicles as they headed north, but he declined to say where they were.

Syrian Democratic Forces seized control of the city on Friday and are now clearing the neighborhoods, looking for militants and bombs. Garver said that a "significant number" of explosive devices were left in the city by IS insurgents as they retreated.

Manbij is a key victory for the SDF and the coalition, because it lies on a major supply route between the Turkish border and the city of Raqqa, the de facto capital of the ISIS group's self-styled caliphate.