Syrian activist groups say airstrikes by the U.S.-led coalition targeting areas controlled by the Islamic State group have killed at least 15 civilians.

The Britain-based Syrian Observatory for Human Rights says 15 people died in strikes on the northern Syrian town of Manbij, which is under attack by U.S.-backed Syrian fighters, and six died in a strike on the nearby village of Tokhar.

The Local Coordination Committees, another activist group, also says 15 people, including women and children, died in Manbij.

There was no immediate comment from the coalition.

The U.S.-backed predominantly Kurdish Syria Democratic Forces have been trying to take control of Manbij from IS. The U.S.-backed fighters have surrounded Mabij and are advancing slowly into the town under the cover of airstrikes by the U.S.-led coalition.