WASHINGTON: ISIS is using civilians as human shields in Syria, a U.S. military official said Friday, at a time when the coalition has been accused of killing dozens of civilians in strikes near Manbij.

The Syrian Observatory for Human Rights accused the coalition of killing 56 civilians, including 11 children, as they fled Tuesday from a village near Manbij, a strategic waypoint between Turkey and the extremist stronghold of Raqqa.

The Britain-based rights monitor relies on a network of pro-opposition activists inside Syria.

The coalition has opened an investigation of the matter.

ISIS "used civilians as human shields and as bait" in an effort to draw the fire of the Syrian Democratic Forces (SDF) toward the civilians, Colonel Chris Garver, a coalition spokesman, said by video conference from Baghdad. The SDF is a U.S.-backed Kurdish and Arab alliance.

Garver said the attack Tuesday came after SDF fighters "observed a large group of Daesh fighters in a convoy who appeared to be readying for a counterattack" against SDF troops in the area. Daesh comes from the Arabic language acronym for ISIS.

"A strike was called in on Daesh. The strike was against both buildings and vehicles." Afterward, the spokesman said, the coalition received both internal and external reports "that there may have been civilians in the area who are mixed in and among the Daesh fighters."

Garver said the first phase of the investigation – what he called a "credibility assessment" – would take no longer than a week and a half. Coalition officials will then determine whether a more intensive inquiry is required.

The spokesman said the extremists had been mounting exceptionally fierce resistance in Manbij, an ISIS bastion.

Fighting has grown more intense as SDF units move into the city, he said, "which is sort of different than what we saw in Ramadi and what we saw in Fallujah," Iraqi cities from which extremists were ousted this year.

"It's a fight like we haven't seen before."

Garver estimated that the SDF had taken back roughly half the city, an area still housing at least 2,000 civilians.

An SDF-allied unit said that on Thursday it had given ISIS fighters 48 hours to leave the city, but Garver said that "as of right now, I'm not confirming that."