US-backed Syrian Kurdish forces say they have entered the fourth stage of their campaign to capture the Islamic State group's de facto capital Raqqa. <br/><br/>The Syrian Democratic Forces (SDF), which include Arab fighters, say they are working to clear the militants out of the Jalab Valley, north of Raqqa. An estimated 300,000 people are in the city. The SDF says, it wants to isolate Raqqa before attacking it.