The raid hit the headquarters of the group in Tawama village in the western countryside of Aleppo province, said the Britain-based Syrian Observatory for Human Rights, which gathers information from a network of sources on the ground.
Most of the fighters killed were Turkish, it said, and there were several also wounded. It was not immediately clear how senior they were.
U.S.-led forces have been striking jihadists in Syria and neighbouring Iraq since mid-2014 and have mainly targeted the militant Islamic State group, but have also killed a smaller number of Nusra Front members in Syria.
Earlier on Wednesday the Combined Joint Task Force leading the air strikes said its forces had hit areas further east near al-Hasaka and the town of Kobani near the Turkish border. (Reporting by Sylvia Westall; Editing by Mark Heinrich)