U.S.-backed Kurdish-led Syrian forces are battling Islamic State (IS) militants north of Raqqa, the extremist group's de facto capital in Syria.
The clashes on November 7 came a day after the Syria Democratic Forces (SDF), a multiethnic opposition alliance led by the Kurdish People's Protection Units (YPG), announced the start of its campaign to retake Raqqa.
The Britain-based Syrian Observatory for Human Rights, a London-based monitoring group, reported heavy fighting north of Raqqa.
The U.S. Central Command said coalition warplanes conducted 16 air strikes in northern Syria, mostly near the area of Ain Issa, where the fighting appeared to be concentrated.
The United States considers the YPG militias the most effective force against IS militants in Syria.
But Turkey is dismayed at the prominent role played in the SDF by the Kurdish YPG militia, which it accuses of having ties with Turkey's outlawed Kurdistan Workers Party (PKK).
Based on reporting by AP and dpa