BEIRUT (Reuters) - U.S-backed Syrian armed groups have captured a number of villages in the first days of an offensive to retake the city of Raqqa from Islamic State militants, a war monitor and a Kurdish source said on Monday.

An Syrian Democratic Forces (SDF) fighter stands near the rubble of a destroyed building, north of Raqqa city, Syria November 7, 2016. REUTERS/Rodi Said

The ground forces are being supported by airstrikes mounted by a U.S.-led coalition, the source said. But he predicted the battle to drive IS from their Raqqa, their main stronghold in Syria, would “not be easy”.

The operation by the Syrian Democratic Forces (SDF), which includes the Syrian Kurdish YPG militia and some Arab groups, began on Saturday and aims to encircle and ultimately capture Raqqa. It should add to the pressure on IS as it faces a major assault on its Iraqi bastion of Mosul.

The attack, named “Euphrates Anger” so far appears focused on areas north of Raqqa, south of the town of Ain Issa, 50 km (30 miles) away.

The Syrian Observatory for Human Rights, an organisation that monitors the multi-sided Syrian conflict, said the SDF forces had so far captured a number of IS positions but there had been “no real progress”.

The Kurdish source said a number of villages had been captured. IS had set off five car bombs as part of their defence, he said.

“It is difficult to put a time frame on the operation at present. The battle will not be easy,” the source said, speaking on condition of anonymity.

The U.S.-led coalition was providing “excellent” air support, he added.

The SDF has been the main ally on the ground in Syria for the U.S.-led coalition against Islamic State in Iraq and Syria, capturing swathes of northern Syria.

The coalition said on Monday the SDF, supported by coalition air and advisory support, had begun the operation to isolate Raqqa.

“The isolation of Raqqa, when complete, will liberate strategically valuable terrain surrounding Raqqa and enable the liberation of the city,” Commander, Lt. Gen. Stephen Townsend said.

The operation will also cut off Islamic State militants from Mosul as Iraqi forces simultaneously try to take back that city, Townsend said.

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

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

Once Raqqa is freed from Islamic State it will be run by a military and a civilian council made up of Raqqa inhabitants, Jihan Sheikh Ahmad, an SDF spokeswoman, said.

This was the post-libertion model of government employed in the northern Syrian city of Manbij, near the Turkish border, after the SDF expelled Islamic State in August.

Townsend said the coalition would consult with its allies throughout the campaign with its allies over plans for Raqqa’s seizure and how it will be governed after that.

A U.S. official told Reuters in Washington there was “no available force capable of taking Raqqa in the near future”, and U.S. officials cautioned the process of sealing off and isolating the city could take two months or longer.