A Syrian Democratic Forces (SDF) fighter stands near rising smoke, north of Raqqa city.

ISIS militants have set off five car bombs targeting US-backed Syrian armed groups attacking Raqqa, a source said on Monday, saying the fight to drive ISIS from its stronghold city would “not be easy.”

The operation by the Syrian Democratic Forces (SDF), including the Kurdish YPG militia, that began on Saturday aims to encircle and ultimately capture Raqqa, adding to the pressure on ISIS as it faces a major assault in Iraq.

ISIS has also drawn heavily on suicide car bombs in its efforts to fend off the assault on Mosul by Iraqi forces.

The attack so far appears focused on areas north of Raqqa near the town of Ain Issa, 50 km (30 miles) away. The Syrian Observatory for Human Rights, an organization that reports on the war, said the SDF forces had so far captured a number of ISIS positions, but there had been “no real progress.”

The Kurdish source said a number of villages had been captured. “Daesh is resorting to attacks with car bombs to a great degree,” the source said.

The SDF has been the main partner on the ground in Syria for the US-led coalition against ISIS, capturing swathes of northern Syria with coalition air support.

Planning for the Raqqa assault has been complicated by factors including the concerns of neighboring Turkey, which does not want to see any further expansion of Kurdish influence in northern Syria.

Additionally, Raqqa is a predominantly Arab city, and Syrian Kurdish officials have previously said it should be freed from ISIS by Syrian Arab groups, not the Kurdish YPG.

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