US Defence Secretary Ashton Carter said Wednesday that the main objective of the coalition fighting the Islamic State group was to destroy its power centres in Syria and Iraq.

He told journalists the aim was to "destroy the ISIL cancer's parent tumour in Iraq and Syria by collapsing its two power centres in Raqa and Mosul," using another name for the jihadist group.

"Second, to combat the metastasis of the ISIL tumour worldwide."