Pro-Ankara fighters taking part in a Turkish offensive on Kurdish-held border towns in north-eastern Syria "executed" at least nine civilians including a female politician on Saturday, a monitor said.
"The nine civilians were executed at different moments south of the town of Tal Abyad," the Syrian Observatory for Human Rights said.
The Kurds said a Kurdish political leader Hevrin Khalaf and her driver were among those killed.
Khalaf, 35, was "taken out of her car during a Turkish-backed attack and executed by Turkish-backed mercenary factions", the political arm of the Kurdish-led Syrian Democratic Forces (SDF) said in a statement.
"This is a clear evidence that the Turkish state is continuing its criminal policy toward unarmed civilians," it added.
Khalaf was the secretary general of the Future Syria Party.
Mutlu Civiroglu, a specialist in Kurdish politics, described her death as a "great loss".
"She had a talent for diplomacy, she used to always take part in meetings with the Americans, the French; the foreign delegations."
Turkey and its allied fighters began the offensive on Wednesday to push back the Kurdish Peoples' Protection Units (YPG), the backbone of the SDF, from its border.
The allied fighters are Syrian former rebels who once fought against President Bashar al-Assad's regime.
But the conflict has morphed since erupting in 2011, and these factions now receive training and funding from Ankara.
Kurdish activists circulated two videos on social media of the killings.
The first, posted on the Twitter account of the Ahrar al-Sharqiya rebel group, shows two people in civilian clothes kneeling on the ground as a fighter next to them announces they have been captured by the faction.
In the second, an unidentified fighter opens fire at a person on the ground wearing civilian attire.
The Observatory confirmed the authenticity of the videos but AFP could not independently verify them.
The deaths brought to at least 38 the number of civilians killed on the Syrian side since start of the assault, according to the Observatory.
It says 81 Kurdish fighters have been killed in the clashes.
Ankara says the YPG is a "terrorist" group with links to Kurdish rebels in Turkey who have been waging an insurgency for three decades.
Later on Saturday, the Syrian National Army, the banner under which the Ankara-allied former rebels fight, said it had ordered commanders to "continuously supervise combatants on the frontlines to prevent any abuse".
Perpetrators of possible abuses "would face the most severe sanctions and be brought to justice for military disobedience", it said.