A US-backed Kurdish and Arab force has announced an operation to capture Raqqa, so-called Islamic State's capital in Syria. <br/><br/> The Syria Democratic Forces say they will be aided by US-led coalition air strikes and will begin in hours. They have also warned civilians to steer clear of areas where IS militants are present.<br/><br/>The coalition of Kurdish and ethnic Arab militias has been gaining ground in areas to the north of the city.