At least 15 civilians, including four children, were killed in a suspected US-led coalition airstrike on Saturday near the Islamic State group's Syrian bastion Raqa, a monitor said.
The Syrian Observatory for Human Rights said at least 17 people were injured in the strike on Heneyda, and that the death toll could rise further because several of the wounded were in serious conditions.
The Britain-based group said the strike was suspected to have been carried out by the US-led coalition fighting IS in Syria and Iraq.
The Observatory, which relies on a network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
Heneyda is around 25 kilometres (15 miles) west of the city of Raqa, the target of a major operation led by a Kurdish-Arab alliance of fighters and backed by the US-led coalition.
The Syrian Democratic Forces (SDF) have for months been advancing towards the city in the north of the country, hoping to encircle it before launching a final assault.
Its forces last month seized the Tabqa military airport from IS, and have entered the complex of the key Tabqa dam, after being airlifted behind jihadist lines by US forces.
They continue to battle for the town of Tabqa, around 40 kilometres (25 miles) west of Raqa, with clashes ongoing on Saturday, the Observatory said.
More than 320,000 people have been killed in Syria since the conflict began with anti-government demonstrations in March 2011.