Pentagon spokesman Rear Admiral John Kirby confirmed that "armed and manned" US aircraft would fly from Irbil, capital of Iraq's autonomous Kurdish region, but declined to offer more details.

The use of Irbil air base reflects the broadening US offensive against the "Islamic State" militants, though attack helicopters already have been flying out of bases in Iraq.

Previously American planes have been attacking "IS" militants out of bases and from aircraft carriers in the region outside Iraq.

The airfield in Irbil would allow fighter jets easier access to the battlefront, a defense official told news agency AFP, speaking on condition of anonymity. Fighters have a shorter range than bombers or surveillance planes, and using a nearby base allows for more time over a target while reducing the need to frequently refuel, the official said.

Kerry gathers Arab allies

US Secretary of State John Kerry continued his diplomatic drive on Thursday in an effort to gather a group of leading Arab countries to "do their share" in US-led efforts to rid the Middle East of the "Islamic State" militants.

Speaking the day after President Barack Obama announced a "relentless" campaign of air strikes against "IS" militants in Iraq and ultimately in Syria, Kerry declined to call the operation a war.

"What we are doing is engaging in a very significant counter-terrorism operation," Kerry told CNN in Jeddah, Saudi Arabia, during a tour of regional allies to drum up support for joint action.

"It's going to go on for some period of time," he warned.

bk/cb (AFP, AP)