Even with the world's attention fixed on the fate of the northern Syrian town of Kobane, General Lloyd Austin said Iraq was the primary battleground for the air campaign.
"Iraq is our main effort, and it has to be," Austin told reporters.
"And the things that we're doing right now in Syria are being done primarily to shape the conditions in Iraq," he said.
In his first press conference since the air campaign was launched in Iraq on August 8, Austin said it would take time before Iraqi government forces were truly effective and declined to say when the army would be ready to stage major offensives to recapture lost territory in Mosul or elsewhere.
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"It's difficult to ...Designate a specific point in time when they'll be able to do this."
Iraqi army troops have suffered a string of setbacks in western Anbar province, raising fears that Baghdad could come under pressure and the airport endangered.
"I feel fairly confident that the airfield is secure and will be secure for the foreseeable future," he said.
It was possible the IS could fire a mortar round at the airfield, but similar incidents occurred during the US occupation of Iraq in 2003-2011 and did not force the closure of the airport, he said.
"This is something that we monitor - we patrol on a routine basis," he said, adding that US Apache helicopters, drone surveillance aircraft and Iraqi forces all help guard the airport.
He said the strikes were a response to IS moving in large numbers around the town, making itself vulnerable to attacks from the air.