The Kurdish People's Protection Units (YPG), which forms the backbone of the Syrian Democratic Forces (SDF), has used child soldiers in the past, according to the United Nations, HRW and other rights groups.
In a report published on Friday, the New York-based watchdog again accused the YPG of recruiting minors.
The SDF's political arm vowed to investigate the HRW claim and hold accountable those responsible for "individual" cases.
"We are against the recruitment of children under any pretext," the Syrian Democratic Council said in a statement.
"What was mentioned in HRW's press release is only irresponsible individual abuse, which does not represent the overall method or strategy of the Syrian Democratic Council."
It said it would study the "allegations," return any child recruits to their families and "hold accountable those responsible for these abuses".
International law prohibits non-state armed factions from recruiting anyone under the age of 18, and enlisting children under 15 is a war crime.
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