BEIRUT (AP) — A Syrian opposition monitor and a news agency linked to the Islamic State group say the extremists have carried out a deadly attack against U.S.-backed forces near the northern city of Raqqa.

The Aamaq news agency said Friday that the attack a day earlier killed 53 members of the Syrian Democratic Forces in the village of Karama.

Nisreen Abdullah, a spokeswoman with the U.S.-backed SDF, said IS is carrying out attacks against the SDF but strongly denied the high number of casualties given by Aamaq.

Rami Abdurrahman, who heads Britain-based Syrian Observatory for Human Rights, said IS fighters used motorcycles in their attack. He says they were able to kill and kidnap dozens of SDF fighters and civilians.

SDF has been on the offensive in Raqqa since June 6.