US-backed Syrian militias not preparing assault on ISIS capital: Spokesperson

US-backed Syrian militias are not preparing an assault on Islamic State's de facto Syrian capital of Raqqa at present, a militia spokesman said, indicating the limited scope of a new offensive in nearby areas where fighting raged on Wednesday.

Movements by the fighters in the US-backed Syria Democratic Forces (SDF) alliance, and leaflets dropped on Raqqa urging its citizens to leave had given rise to speculation that they were about to attack the city.

Driving Islamic State from Raqqa city would be a major achievement in the US-led campaign against the group that controls wide areas of Syria and Iraq. But a spokesman for the SDF indicated it was not imminent.

"The current battle is only to liberate the area north of Raqqa. Currently there is no preparation ... to liberate Raqqa, unless as part of a campaign which will come after this campaign has finished," spokesman Talal Silo said.

Leaflets dropped on Raqqa city had urged residents to flee. The Syrian Observatory for Human Rights, a group that reports on the war using an activist network on the ground, says hundreds of families have left.

Syria experts however doubt that the SDF is ready for an attack on Raqqa city. Its most powerful component is the Syrian Kurdish YPG militia, and Syrian Kurdish officials have previously said Arab groups must be the ones to lead any assault on the predominantly Arab city of Raqqa.

The Observatory said there were clashes in the countryside south of Tel Abyad and around Ain Issa, a town about 60 km north west of Raqqa on Tuesday and Wednesday. It also said the SDF campaign has been supported by heavy air strikes from the US-led coalition against Islamic State.

The SDF launched an operation at 2pm (1100 GMT) on Tuesday to recapture land between the SDF stronghold in Tel Abyad near the Turkish border and the Islamic State's defacto capital in Syria in Raqqa - but there are no plans to advance on the city itself yet.

Aided by US-led air strikes, the YPG has driven Islamic State from wide areas of northern Syria over the last year or more, though its advances have recently slowed.

Syrian Kurdish groups have previously said an attack on the predominantly Arab city of Raqqa should be led by Arab militias. Syria experts say the SDF's Arab groups are not yet ready for such an attack, however.

Islamic State's territory in Iraq and Syria has shrunk significantly from its peak. The group is also being targeted in a separate campaign by the Syrian military and its allies, including Russia.