A spokesman for the U.S.-backed forces fighting Islamic State militants in Syria says the "final" battle to uproot the extremists from the northern city of Raqqa is under way.

Mustafa Bali, the spokesman for the Kurdish-led Syrian Democratic Forces, said on October 14 that the battle could take hours or days.

The Syrian Democratic Forces (SDF) is an alliance of Arab and Kurdish militias dominated by the Syrian Kurdish YPG militia.

The loss of Raqqa, once the de facto capital of its self-proclaimed caliphate, would deal a huge blow to the militant group.

Bali says IS militants are putting up a desperate resistance in a number of neighborhoods in the city.

The battle for Raqqa began in June, with heavy street-by-street fighting amid intense U.S.-led coalition airstrikes and shelling. The battle has dragged in the face of stiff resistance from the militants and civilians trapped in the city.

An activist group that reports on Raqqa, Raqqa Is Being Slaughtered Silently, said on its Facebook page on October 14 that dozens of buses had entered Raqqa city overnight, having traveled from the northern Raqqa countryside.

The Britain-based Syrian Observatory for Human Rights said Syrian Islamic State fighters and their families had already left the city, and buses had arrived to evacuate remaining foreign fighters and their families. It did not say where they would be taken to.

Based on reporting by Reuters and AP