U.S. Defense Secretary Ash Carter arrived in Iraq Saturday to meet with his commanders and assess progress in the opening days of the operation to retake the northern city of Mosul from Islamic State militants.

His unannounced visit comes two days after a U.S. service member was killed outside Mosul, underscoring the risk to American troops as they advise Iraqi forces. And it comes on the heels of Carter announcing there "is an agreement in principle" for Turkey to help in the Mosul battle, and that friction between Turkey and Iraq can be worked out.

This is Carter's third trip to Iraq this year, and he has overseen the steady increase in the number of U.S. forces deployed and the growth of America's effort to train and advise Iraqi troops.