“They are in a gray zone, but in a civil war if you are not willing to talk to factions in the gray zone, you’ll have precious few people to talk to,” said Robert S. Ford, a former United States ambassador to Syria now at the Middle East Institute.

“I do not advocate giving any material support to Ahrar, much less lethal material assistance, but given their prominence in the northern and central fronts, they will have a big role in any peace talks, so we should find a channel to begin talking to them,” he said.

Ahrar al-Sham cooperates with the Syrian affiliate of Al Qaeda and has welcomed former associates of Osama bin Laden. While its leaders say they seek to create a representative government, they avoid the word “democracy” and say Islam must guide any eventual state.

Similar questions about how much to engage with Islamist forces on issues of mutual interest have consumed American policy makers since the Arab Spring, and in the nuclear talks with Iran.