Warplanes believed to be part of a U.S.-led alliance struck Nusra Front positions in northern Syria on Friday following an attack by the al Qaeda-linked group on Western-backed rebels in the area, a group tracking the war said.

The Syrian Observatory for Human Rights, a UK-based group that reports on the war, said the air strikes hit Nusra Front positions near the town of Azaz, north of Aleppo, Daily Mail reported.

Overnight, the Nusra Front had launched an attack in the area targeting rebels including a group said by opposition sources to have been trained under a U.S.-led programme to build a force to fight Daesh, the Observatory reported.

The rebel group, known as Division 30, said Nusra Front fighters attacked its headquarters at 4.30 am. Five members of Division 30 were killed as they held off the attackers, it said.

The group has accused the Nusra Front of abducting its leader and several other members earlier this week.