The United States-led coalition campaign against the Islamic State group in Iraq and Syria has successfully completed its first 'phase' of operations, a US military spokesman said on Wednesday.

The coalition is working through three main steps as it wages its 20-month-old fight against the IS group, Baghdad-based spokesman Colonel Steve Warren said.

"Our enemy has been weakened and we now are working to fracture him. Phase one of the military campaign is complete," Warren told Pentagon reporters, noting that this initial step was to 'degrade' the IS group by stopping it from making additional territorial gains.

"We are now in phase two, which is to dismantle this enemy," he added.

Warren said the final phase of the campaign is to ensure the IS group is dealt a lasting defeat, primarily by enabling local forces to prevent a resurgence of jihadist influence.