Fighters from the Syrian Democratic Forces, a US-backed Kurdish-Arab alliance, Sunday entered an military airport held by the Islamic State jihadist group in northern Syria, a spokesman said.

‘The SDF has taken control of more than 50 percent of Tabqa military airport. Fighting is ongoing inside the airport and its surroundings and full control of the airport is expected within the next few hours,’ Talal Sello told AFP.

The Syrian Observatory for Human Rights monitor said IS forces had withdrawn from the airbase under heavy artillery fire and US-led coalition air strikes.

The jihadist group has held the military airport since August 2014, and it carried out one of its worst mass killings there, of some 200 government soldiers.

The SDF is pressing an assault on the town of Tabqa, the nearby Tabqa dam and the airbase in preparation for an attack on IS's de facto Syrian capital Raqa city.

Raqa lies around 35 miles (55 kilometres) to the east of Tabqa and is one of IS's last remaining strongholds in Syria.

The jihadist group is under pressure on several fronts, with government forces backed by Russia attacking it elsewhere, along with rebel fighters supported by Turkish firepower.