John Kerry seeks US-led coalition against Islamic State

US Secretary of State John Kerry has called for a global coalition to combat Islamic State militants and their "genocidal agenda".

His remarks came as Saudi Arabia's King Abdullah warned that the West would be the group's next target unless there was swift action to stem its advances through Iraq and Syria.

Australia, meanwhile, joined Britain in heightening warnings against terrorism, with both governments concerned that their citizens who have joined Islamic State may return with plans to attack on their home soil.

Writing in The New York Times, a week before a Nato summit in Wales, Kerry urged "a united response led by the United States and the broadest possible coalition of nations".

He said he and Defence Secretary Chuck Hagel would meet European counterparts on the sidelines of the summit to enlist assistance, and then travel to the Middle East to build support "among the countries that are most directly threatened".

The Saudi king's comments also appeared aimed at drawing Washington and Nato forces into a wider fight against extremists in the region. Saudi Arabia has backed rebels fighting in Syria, but is concerned that extremists there could turn their weapons on the kingdom.