Also killed on a main road through the territory was Hervin Khalaf, the head of a Kurdish political party. Kurdish officials accused Turkish-allied Arab fighters of attacking her car, a charge a spokesman for the Arab fighters denied.

Mr. Rabah said his group killed nine Kurdish fighters, he said, without specifying whether the two prisoners were among them.

When asked about the video showing the Turkish-allied fighters shooting the bound prisoner, Mr. Rabah grew nervous and said he had been trying to stop them from shooting.

“Everything happened so fast,” he said

In the video, the fighters call the captives “pigs” and shout “God is great!” as two of them shoot the bound captive.

Mr. Rabah said that the second prisoner’s identification card linked him to a Kurdish security force, and that the Turkish-backed group had killed him after he tried to flee.

“You know in the law of war, it is O.K. to kill anyone who is a threat,” he said.

But a video published by the militant group on Twitter shows that same prisoner sitting passively on the side of the road, dabbing blood from his nose as if he has just been hit.

The video with the fighters shooting the bound prisoner shows three other prisoners, including the one in the military uniform. But in a later post on the group’s Twitter feed announcing its captives, there are only two in the photo. The prisoner in the uniform is not among them.