Washington is in "close contact" with Ankara over the assault launched by American-backed Kurdish-Arab forces on the Islamic State group's bastion Raqa in Syria, a senior US official said Sunday.

"We are in close close contact with our Turkish allies and that is why the chairman of joint chiefs is in Ankara today," Brett McGurk, President Barack Obama's envoy to the US-led coalition battling the jihadists, told a news conference in the Jordanian capital Amman.

"We want this to be as coordinated as possible, recognising that there will be a mix of forces on the field and that many of those forces of course do not see eye to eye, but they do share a very common and still very lethal enemy," he said of IS.