Activists say predominantly Kurdish fighters are now in control of most of a stronghold of the Islamic State group in northern Syria after a push under the cover of airstrikes by the U.S.-led coalition.

The Britain-based Syrian Observatory for Human Rights says Syria Democratic Forces are in control of most of the town of Manbij amid fighting in northern neighborhoods and the town's center.

Mustafa Bali, a Syria-based Kurdish activist, said Saturday that IS still holds some areas in Manbij, including the major northwestern neighborhood of Sarab.

Bali said "it's a matter of time" before SDF fighters capture the town.

If Manbij is captured by SDF, it will be the biggest strategic defeat for IS in Syria since July 2015, when the extremists lost the border town of Tal Abyad.