US and Arab warplanes pounded ISIS positions in Syria overnight, targeting fighters, artillery and combat equipment in nine airstrikes, the US military said.

The latest attacks came amid news of the beheading of a fourth western hostage, British aid worker Alan Hedding, whose videotaped execution was posted on the Internet.

US bombers and fighter jets also carried out five airstrikes against the ISIS group in Iraq.

The Syrian Observatory for Human Rights said 35 militants were killed in the airstrikes overnight in Syria.

The United States was joined by Jordanian, Saudi and UAE warplanes in the attacks inside Syria, which involved a mix of fighter aircraft and drones.

Four of the airstrikes were around Kobane, a town along the Syrian border with Turkey that has been under attack by IS fighters.

Two of those strikes hit a large ISIS unit, a building, three artillery positions and an ISIS vehicle east of Kobane, which also is known as Ain al Arab, US Central Command said.

South of Kobane, a small ISIS unit, an armored personnel carrier, and a vehicle were struck in two other air attacks.

The warplanes also struck an ISIS depot and logistics complex in Hasakeh, where 30 militants were reported killed.

Centcom said strikes also damaged the Taqba Airfield southwest of Raqa and another destroyed an artillery piece west of there.

Two airstrikes southeast of Deir Ezzor destroyed a tank and a vehicle.