Eight of the civilians killed were children and five were women, the Observatory said.
The United States has said it takes reports of civilian casualties seriously and says it has a process to investigate any reports of such deaths.
The Observatory, which gathers its information from a network of contacts on the ground, said 72 members of al Qaeda’s Syria wing Nusra Front were also killed in the air strikes, which started on September 23.
The United States has said it has targeted the “Khorasan Group” in Syria, which it describes as a grouping of al Qaeda veterans under the protection of Nusra Front. Most analysts and activists do not differentiate between the groups in this way.
According to the United Nations, around 200 000 people have been killed in Syria’s conflict, which is in its fourth year. — Reuters