"Syrian pro-regime forces initiated an unprovoked attack against well-established Syrian Democratic Forces (SDF) headquarters" on February 7, said a statement released by the US Central Command early on Thursday, presstv.ir reported.
The statement noted that coalition service members were accompanying the so-called SDF while they came under the attack.
Reuters later cited a US official as saying on condition of anonymity that over 100 pro-government forces had been killed in the assault.
The US has around 2,000 military personnel in northern Syria supporting the SDF, which is an alliance of militias in northern and eastern Syria and largely dominated by the Kurdish People’s Protection Units (YPG).
It added that the coalition "conducted strikes against attacking forces to repel the act of aggression against partners engaged in the Global Coalition's defeat-Daesh mission."
The statement also claimed that the US-led attack was "in self-defense."
The US and its allies have been bombarding what they call Daesh positions inside Syria since September 2014 without any authorization from the Damascus government or a UN mandate.
The strikes, however, have on many occasions resulted in civilian casualties and failed to fulfill their declared aim of countering terrorism.
The Syrian Foreign Ministry has on several occasions written to the UN, complaining that the US is flagrantly violating the sovereignty of Syria by targeting residential neighborhoods.