Pro-Ankara fighters taking part in a Turkish offensive on Kurdish-held border towns in northeastern Syria have "executed" nine civilians, a monitor said on Saturday.

"The nine civilians were executed... in the south of the town of Tal Abyad," the Syrian Observatory for Human Rights said.

The Kurds said in a statement that a female Kurdish party official and her driver were among those killed.

It also emerged that the commander of the Kurdish-led Syrian Democratic Forces had warned the US that "You are leaving us to be slaughtered."

CNN reported that Gen. Mazloum Kobani Abdi told the Deputy Special Envoy to the Global Coalition to Defeat ISIS, William Roebuck: "You are not willing to protect the people, but you do not want another force to come and protect us. You have sold us. This is immoral."

He added: "I need to know if you are capable of protecting my people, of stopping these bombs falling on us or not. I need to know, because if you're not, I need to make a deal with Russia and the regime now and invite their planes to protect this region."

The comments were contained in an internal US government readout of a meeting on thursday, which was obtained by CNN.