Turkish-backed Syrian opposition fighters evacuate an injured fellow combatant Saturday in Tel Abyad, Syria. Credit: Aref Tammawi | DHA via AP

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 five-day-old effort to seize territory controlled by the Kurds allies 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 may have committed war crimes.

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

A separate video shows the fighters crowding round a black, bullet ridden SUV that had apparently come under a hail of gunfire before being forced to stop. As the fighters step over the body of a dead man in civilian clothing to reach inside the vehicle, a female voice is briefly heard coming from the back seat.

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

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

A Turkish newspaper, Yeni Safak, trumpeted her 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 big setback for the group.

According to the Syrian Observatory for Human Rights, a monitoring group, a total of nine civilians were executed on 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 manning the roadblock, show captured men surrounded by fighters on the side of the road.

Ahrar al-Sharqiyeh is composed of fighters mostly from the eastern province of Deir al-Zour, much of which is currently 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 filmed killing and others that may have occurred off camera almost certainly constitute a war crime, according to international law, and may breach one of the conditions 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 else 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 group, as a terrorist group on its doorstep and a threat to its national security.

Washington Post writer Asser Khattab contributed to this report.