BEIRUT — Videos posted on social media showing at least one execution-style killing have called into question the discipline of the soldiers engaged in Turkey’s near week-long effort to seize territory controlled by Kurds in northeastern Syria.

The most gruesome and explicit of the videos shows Turkish-allied Syrian fighters pumping bursts of automatic fire into the body of a bound man lying on the side of a desert road as a gunman shouts to his comrades to take his phone and film him doing the shooting. Another trembling, handcuffed man crouches on the opposite side of the road as the shooting erupts. “Kill them,” one man is heard shouting.

The video is one of a series of photographs and videos posted on Twitter accounts of the Turkish-backed rebel groups and circulated by the U.S.-allied Syrian Democratic Forces that suggest some of the Syrian rebels participating in Turkey’s offensive to capture territory in Syria might have committed war crimes.

Defense Secretary Mark T. Esper spoke on "Face the Nation" Oct. 13 about reports of Turkish-backed fighters committing acts of violence in Syria. (Reuters)

The Turkish army is leading the incursion, but is relying heavily on Syrian rebels to drive the Kurdish-led SDF away from Turkey’s border.

A separate video shows fighters crowding around a black, bullet-riddled SUV that had apparently come under a hail of gunfire before being forced to stop. The fighters step over the body of a man in civilian clothing to reach inside the vehicle.

“Another fleeing pig has been liquidated by the hands of the National Army,” one of the fighters says as the others clamor to be filmed. “He was fleeing in an armored car.”

A female voice is briefly heard coming from the back seat.

What happened next is unclear, but the Kurdish-led SDF say the woman in the car was Kurdish politician Hevrin Khalaf, whose body was found later in a nearby morgue. Khalaf was the secretary general of the newly established Future Party of Syria.

U.S. Defense Secretary Mark T. Esper, speaking on CBS’s “Face the Nation,” noted that the videos could not be independently confirmed. But, he said, if true, “they would be war crimes.”

A Turkish newspaper, Yeni Safak, trumpeted Khalaf’s killing as a “successful operation” against a politician affiliated with the “terrorist” People’s Democratic Union, the Kurdish political party that runs northeastern Syria. 

The newspaper said she had been “neutralized” in the operation and described her death as a major setback for the group.

The Syrian Observatory for Human Rights, a monitoring group, said nine civilians were executed Saturday at the roadblock to the south of the town of Tal Abyad. Several other photos and videos posted by the Ahrar al-Sharqiya rebel group, which was apparently among those involved in staffing the roadblock, show captured men surrounded by fighters on the side of the road.

Ahrar al-Sharqiya is composed of fighters mostly from the eastern province of Deir al-Zour, much of which is controlled by the SDF.

The Syrian National Army, an umbrella group uniting a number of Syrian rebel factions, condemned the killing in a statement and said it had launched an investigation into what it said represented a violation of “the standards and values that we commit to.”

The killing and others that might have occurred off camera almost certainly constitute a war crime, under international law, and could breach one of the conditions President Trump set for allowing the Turkish offensive to go ahead unhindered by U.S. troops in the area. In a tweet last week, he cautioned the Turks not to undertake any “unforced or unnecessary fighting” or they would face measures against their economy and currency.

Turkey views the Syrian Kurdish forces, which were key U.S. allies in the fight against the Islamic State, as a terrorist group on its doorstep and a threat to its national security.

Asser Khattab in Beirut contributed to this report.