"What apparently happened was that the Iraqi forces just showed no will to fight," Carter told CNN's Barbara Starr. "They were not outnumbered. In fact, they vastly outnumbered the opposing force, and yet they failed to fight, they withdrew from the site, and that says to me, and I think to most of us, that we have an issue with the will of the Iraqis to fight ISIL and defend themselves."
Carter's remarks are the strongest yet from any Obama administration official speaking on the record since the last week's events when Ramadi fell. The U.S. has sped up the shipment of some arms to help boost Iraqi forces as ISIS has recently taken more territory, but the U.S. defense chief said Iraq's military needs to step up.
"We can give them training, we can give them equipment -- we obviously can't give them the will to fight," Carter said. "But if we give them training, we give them equipment, and give them support, and give them some time, I hope they will develop the will to fight, because only if they fight can ISIL remain defeated."
Carter said it was "very concerning" the local forces showed little willingness to fight, as they are the ones who will be charged with fighting, winning and holding the territory against ISIS.
In the wake of ISIS advances, some -- including Senate Armed Services Committee Chairman Sen. John McCain --have called for more American forces on the ground in Iraq. Currently, there are about 3,000 U.S. military personnel training Iraqi forces, but they are not near combat areas.
Some experts have called for putting some American forward air controllers who would be near the fighting to help better pinpoint the targets for coalition airstrikes. Carter told CNN he has not forwarded a recommendation for them to the White House.
"If there comes a time when we need to change the kinds of support we're giving to the Iraqi forces, we'll make that recommendation. But what happened in Ramadi was a failure of the Iraqi forces to fight," Carter repeated.
"So our efforts now are devoted to providing their ground forces with the equipment, the training and to try to encourage their will to fight, so that our campaign enabling them can be successful both in defeating ISIL and keeping ISIL defeated in a sustained way," he said.
Carter said the U.S. will continue efforts to provide equipment and training so Iraqi Prime Minister Haider al-Abadi and his government can wage an effective battle against ISIS, but the U.S. support alone would be no guarantee of success.
"That is why I think we need to redouble our efforts to get -- hasten this delivery of equipment to them, their training to support Prime Minister Abadi," he said. "We can't make this happen by ourselves, but we can assist it to happen, and we are counting on the Iraqi people to come behind a multi-sectarian government in Baghdad."
"But again, we can't make that happen. We can only help create the conditions in which it will happen," Carter said.
Carter's full interview will air on CNN's "State of the Union" at 9 a.m. and 12 p.m. on Sunday.