Turkish-backed militants fighting in Syria killed at least two Kurdish prisoners on Saturday, one of them lying on the side of the road with his hands bound behind his back, according to a video from the scene and a fighter who said he witnessed the killings.
In the video, two of the fighters fire bullets at close range into the man with his hands tied, apparently to make sure he is dead. The other prisoner appears in the video alive and wearing a military uniform, but he is missing from the militant group’s later social media posts about its captives.
“The guy in the military outfit was neutralized,” said Al-Harith Rabah, a media activist with the rebel group who was at the scene.
The killing of two Kurdish captives by Arab fighters is a powerful illustration of the forces unleashed by President Trump’s decision to pull back American troops shielding the Kurds. The move cleared the way for a Turkish military incursion into northeastern Syria against a Kurdish-led militia formerly backed by the United States — one that had been key in wresting territory from Islamic State.
The episode offers the first indication of the sectarian violence between the region’s Kurds and the Arab rebels supported by Turkey that the decision could unleash. The new hostilities have also displaced tens of thousands more people in northeastern Syria and ignited fears that thousands of ISIS fighters could escape camps and prisons controlled by the Kurdish-led militia.
The killings occurred early Saturday near the Syrian town of Tal Abyad after the rebel group Ahrar al-Sharqiyeh, which had entered Syria from Turkey, took control of a road near the border and began stopping passing cars, according to Mr. Rabah, the media activist.
When military vehicles belonging to the Kurdish-led militia passed, the rebels would stop them and at times, clashes would erupt, he said. His group killed nine Kurdish fighters, he said, without specifying whether the two prisoners were among them.
When asked about the video showing rebels shooting the bound prisoner, Mr. Rabah grew nervous.
“I was trying to stop them,” he said. “Everything happened so fast.”
But in the video, he can be heard calling the captives “pigs” as other rebels shout “God is great!” and shoot the bound captive.
Mr. Rabah said the second prisoner’s identification card said he belonged to a Kurdish internal security force, and that the rebels 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 group on Twitter shows that same prisoner in a military uniform sitting passively on the side of the road, dabbing blood from his nose as if he has just been hit.
The video showing the fighters shooting the bound prisoner also shows three other prisoners, including the one in the military uniform. But in a later post on the rebel group’s Twitter feed announcing its captives, there are only two in the photo.
Turkey’s incursion into northeastern Syria comes more than eight years into a civil war that has shattered the country and pulled in Russia, Iran, Turkey, and the United States, all of which have forces on the ground backing their Syrian allies.
The United States has long backed a Kurdish-led militia in the country’s northeast called the Syrian Democratic Forces, or S.D.F., which played a key role in the battle against the jihadists of the Islamic State.
As the jihadists lost territory, the S.D.F., expanded to effectively control more than one-quarter of Syria’s land.
The rise of Kurdish autonomy across its southern border, enabled by United States military aid, angered Turkey, which considers the S.D.F. a security threat. President Recep Tayyip Erdogan had threatened to send Turkish forces into Syria to root out Kurdish militants long before Mr. Trump acquiesced to the idea on Sunday.
To prepare for the incursion, Turkey supported groups of predominantly Arab rebels in Syria who also oppose the S.D.F. In videos posted as the incursion began, the rebels often referred to the Kurdish fighters as pigs and invoked religious battle language to justify the fight against them.
Previously, the roughly 1,000 United States troops posted in northeastern Syria formed a buffer between the Arab rebels and Kurdish fighters, running daily patrols to keep them apart.
But that buffer collapsed with the Turkish incursion because Turkey began bringing Arab rebels across its border directly into the Kurdish-controlled zone. It was one such group that carried out the killings on Saturday.