U.S.-backed Syrian rebels say al-Qaida militants have seized their bases and stolen weapons in a series of raids in the northern Idlib province.

Division 13 of the Free Syrian Army said on Twitter Sunday that the al-Qaida-affiliated Nusra Front occupied and looted its posts late the night before.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, says Nusra seized anti-tank missiles, armored vehicles, a tank, and other arms from the division, which has received weapons, training, and money from the U.S. government. It says Nusra and an allied militia detained 40 fighters from Division 13.

Both the FSA and the Nusra Front are fighting to overthrow President Bashar Assad.