US Secretary of State John Kerry said on Sunday it was ultimately up to Iraqis to fight back against the Islamic State group advancing in Iraq despite US-led airstrikes.

"Over time we believe that the strategy will build, the capacity will build, Daesh will become more isolated," he said, referring to the jihadist IS by its acronym in Arabic.

"But ultimately it is Iraqis who will have to take back Iraq. It is Iraqis in Anbar who will have to fight for Anbar."