Beirut (AFP) - US-backed Syrian forces said Sunday they have established a military council to push Islamic State group fighters out of their northern bastion of Al-Bab after ousting the jihadists from Manbij.

"We announce... the creation of the Al-Bab Military Council" tasked with driving IS from the town in Aleppo province, said the Syrian Democratic Forces alliance of Kurdish and Arab fighters in a statement, two days after driving the last jihadists from Manbij.

The last remaining IS fighters abandoned the city of Manbij near the Turkish border on Friday after a rout that the Pentagon said showed the extremists were "on the ropes".

The retreat from the city which IS captured in 2014 was the jihadists' worst defeat yet at the hands of the SDF alliance backed by US air power.

Al-Bab is around 50 kilometres (30 miles) southwest of Manbij, and also in the battleground province of Aleppo.

In Sunday's statement, the SDF said "we promise to our people that we will strike to liberate Al-Bab@" and the region around it.

They also called on the US-led coalition that has been battling IS in Syria and neighbouring Iraq "to back us in our struggle to liberate our land and our brothers from the Daesh terrorists".

The battle for Manbij -- a key supply route for the jihadists between the Turkish border and their self-declared capital in Raqa -- lasted more than two months.

As they fled, the jihadists took hundreds of civilians with them to use as "human shields" but later released many of them.