Around 100 fighters with the so-called Islamic State group have surrendered in Syria's Raqqa in the last 24 hours and were "removed from the city", a spokesman for the US-led coalition against the jihadist group has told Reuters.

"We still expect difficult fighting in the days ahead and will not set a time for when we think (Islamic State) will be completely defeated in Raqqa," Colonel Ryan Dillon said in an emailed statement.

It did not say how they had been removed or where the fighters had been taken.

The Syrian Kurdish YPG militia has said the jihadist group is on the verge of defeat in the city, adding that the wartorn city may finally be cleared of IS forces this weekend.

"The battles are continuing in Raqqa city. Daesh (another term for the Islamic State group) is on the verge of being finished. Today or tomorrow the city may be liberated," YPG spokesman Nouri Mahmoud told Reuters by telephone.

The YPG dominates the US-backed Syrian Democratic Forces, an alliance of Arab and Kurdish militias that has been battling since June to defeat IS in Raqqa.

The city has served as the jihadist group's de facto capital in Syria.

The British-based Observatory said Syrian IS 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.

The Observatory said the evacuation was taking place according to a deal reached between the SDF and the US-led coalition on the one hand, and IS on the other.

During the more than six-year Syrian war, the arrival of buses in a conflict zone has often signalled an evacuation of combatants and civilians.

In August, IS fighters agreed to be evacuated from a Lebanon-Syria border area, the first time the militants had publicly agreed to a forced evacuation from territory they held in Syria.

Civilians have been making perilous journeys to escape IS-held areas as SDF forces advance.

The SDF says it helps transport them away from the fighting after they flee.

The offensive to drive IS out of Raqqa, which it seized in 2014, has long outlasted initial predictions by SDF officials who said ahead of a final assault in June that it could take just weeks.