"We won't hold back from supporting capable partners in opportunistic attacks against ISIL, or conducting such missions directly whether by strikes from the air or direct action on the ground," Carter said in testimony before the Senate Armed Services committee, using an alternative name for the militant group.

Carter pointed to last week's rescue operation with Kurdish forces in northern Iraq to free hostages held by ISIS.

Carter and Pentagon officials initially refused to characterize the rescue operation as U.S. boots on the ground. However, Carter said last week that the military expects "more raids of this kind" and that the rescue mission "represents a continuation of our advise and assist mission."

This may mean some American soldiers "will be in harm's way, no question about it," Carter said last week.

After months of denying that U.S. troops would be in any combat role in Iraq, Carter late last week in a response to a question posed by NBC News, also acknowledged that the situation U.S. soldiers found themselves in during the raid in Hawija was combat.