The U.S.-led coalition said it had no information on the report by the war monitor, but would look into it.
According to Reuters, the coalition is providing air support to the Syrian Democratic Forces (SDF), an alliance which includes the Syrian Kurdish YPG militia and some Arab groups, in an operation to drive Islamic State from its de facto capital in Syria.
The attack hit al-Heesha, about 40 km (25 miles) north of Raqqa, said the British-based monitor.