The US-led coalition fighting the Islamic State group is tracking 100-200 jihadists after they used human shields to flee a key city in Syria, a US official said Tuesday.

IS fighters had controlled Manbij in northern Syria since early 2014, and the city had become a vital waypoint for the group as they funneled foreign fighters from the Turkish border to other parts of their self-declared caliphate.

But as it became clear that US-backed Syrian Democratic Forces (SDF) would re-capture the city last week, some 100-200 jihadists fled in cars and trucks, Baghdad-based coalition spokesman Colonel Chris Garver told reporters in a video call.

Unlike in Fallujah in Iraq, where local and coalition forces destroyed about 175 IS vehicles as they fled that city, the SDF outside Manbij did not open fire on outgoing cars.

"Civilians were observed in the convoy intermingled with fighters in every vehicle," Garver said.

"We have repeatedly mentioned the care that our partnered forces were taking to avoid civilian casualties and collateral damage, so the partnered forces on the ground did not engage the convoy."

He said the coalition was tracking those IS fighters but declined to elaborate, saying only that they went north and split up. Parts of the Syrian border with Turkey are still controlled by IS.

"We're keeping track of them. I don't want to talk too much about that. It's an ongoing operation," Garver said.

Garver said those in the vehicles were likely a mix of IS fighters, civilian hostages and people traveling willingly with the jihadists, such as family members.

"We had to treat them all as noncombatants. We didn't shoot. We kept watching," he told reporters.

Hundreds of the civilians from the convoy were released on Saturday and others escaped.

He also noted that IS forces had apparently repeatedly tried to put civilians in harm's way during the SDF operation to free Manbij.

"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," he said.

The jihadists, who have suffered a string of losses in Syria and Iraq, have often staged mass abductions when they come under pressure to relinquish territory they hold.