Lt. Col. Fares al-Bayyoush, the former aviation engineer who now heads the Fursan al-Haq Brigade, acknowledged that his men had fought alongside the Nusra Front because they needed all the help they could get.

Sometimes, he said, that help comes in forms only a jihadi group can provide. He cited the rebel takeover of the northern town of Khan Sheikhoun, saying that the rebels were unable to take out one government position until the Nusra Front sent a suicide bomber to blow it up.

In another town nearby, Nusra sent four bombers, including an American citizen.

“We encourage them actually,” Mr. Bayyoush said with a laugh. “And if they need vehicles, we provide them.”