John Kerry downplays talk of 'war' against Islamic State militants
US Secretary of State John Kerry makes a statement on the execution of American journalist Steven Sotloff during a ceremony at the State Department in Washington, DC on September 3, 2014 (AFP Photo/Jim Watson)

US Secretary of State John Kerry warned against "war fever" on Thursday and said the new American campaign against the so-called "Islamic State" should be understood as a counter-terrorism mission.

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.

"If somebody wants to think about it as being a war with ISIL, they can do so, but the fact is it's a major counter-terrorism operation that will have many different moving parts."

Separately, CBS reporter Margaret Brennan tweeted that Kerry had told her: "I don’t think people need to get into war fever on this."

Meanwhile, the Pentagon announced that US warplanes would start using a base outside the Iraqi Kurdish capital Arbil, having previously been operating from airbases and carriers outside the country.