US President Barack Obama laid out his administration's strategy today for combating the Islamic State organisation in Iraq and Syria by forming a broad international coalition against the group.

"We're moving ahead with our strategy to degrade and ultimately destroy this terrorist organisation," Obama said in his weekly radio address.

Obama insisted that the conflict with the Islamic State "can't be America's fight alone".

"What's needed now is a targeted, relentless counter-terrorism campaign against ISIL that combines American air power, contributions from allies and partners, and more support to forces that are fighting these terrorists on the ground. And that's exactly what we're doing," he added.

In an address to the nation on Wednesday night, Obama announced his decision to authorise US air strikes against Islamic State targets in Syria for the first time, and to add to targeted areas in Iraq.

Obama is leading an effort to form a coalition of Western allies and Gulf Arab states to take on the extremist group, whose savage methods have included beheading two American journalists.

"We have to use our power wisely. And we have to avoid the mistakes of the past," said Obama, who wants to avoid a repeat of the Iraq war and has vowed not to send large numbers of US combat troops there.

Obama met with NATO allies last week in Wales and later this month will hold a leaders security conference at the UN general assembly in New York, aimed at gaining commitments from nations willing to join the coalition.

Saudi Arabia has agreed to host a training mission for those Syrian rebels deemed moderate by the United States.