The strikes hit a northern town controlled by the Islamic State jihadist group, said the Syrian Observatory for Human Rights.
“Airstrikes by international coalition fighter jets after midnight (Thursday morning) on the town of Ghandoura killed at least 15 civilians and wounded dozens,” said the monitoring group’s head, Rami Abdel Rahman.
The town lies 23 kilometres (14 miles) northwest of Manbij, a strategic waypoint between Turkey and the jihadist stronghold of Raqa.
The US military acknowledged Thursday that there may have been civilian deaths.
US Central Command, which oversees US military operations in the Middle East, said it had “initiated an assessment following internal operational reporting that a strike today near Manbij, Syria may have resulted in civilian casualties.”
The strikes came a day after the coalition opened a formal investigation to determine whether its air strikes last week near Manbij claimed civilian lives.
The Observatory and local residents said strikes on July 19 had killed dozens of civilians, including children.
After examining “internal and external information”, the coalition determined that there was sufficient credible evidence of civilian victims to open a formal inquiry, said spokesman Colonel Chris Garver on Wednesday.
Garver said last week that the jihadists had been mounting exceptionally fierce resistance in Manbij, which is being besieged by US-backed Syrian forces.
The main Syrian opposition group had urged the US-led coalition to suspend its bombardments following the strikes.
Amnesty International has called on the coalition to redouble its efforts to prevent civilian deaths and to investigate possible violations of international humanitarian law.