Defense Secretary Ash Carter has arrived in Iraq, saying U.S. and coalition forces will use the newly retaken air base in Qayara (kee-AH'-rah) as a staging hub, as Iraqi security forces move closer to the long-awaited battle to recapture Mosul from Islamic State militants.

Carter, in an unannounced visit, says U.S. advisers are prepared to accompany Iraqi battalions, if needed, as they move toward Mosul.

A senior defense official says it's unclear when U.S. advisers would begin accompanying the battalions, but it could be in the coming weeks and months. The official was not authorized to discuss the matter publicly, so spoke anonymously.

Carter laid out the U.S. vision for Qayara for the first time, describing its recapture as a key strategic victory.