The Islamic State group's takeover of Ramadi is evidence that Iraqi forces do not have the "will to fight", US defence secretary Ash Carter said.
In an unusually candid assessment, Mr Carter said Iraqi forces vastly outnumbered the opposition in Ramadi but failed to fight and pulled back from the provincial capital in central Iraq.
Mr Carter said the United States can provide training, weapons and air support in the fight against the Islamic State extremists but he said the Iraqi forces will have to show a willingness to fight for their country.
He made his comments in an interview that aired today on CNN's State Of The Union show.
The Iraqis left behind large numbers of US-supplied vehicles, including several tanks.
"What apparently happened is the Iraqi forces just showed no will to fight. They were not outnumbered," Mr Carter said. "In fact, they vastly outnumbered the opposing force. 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."
The fall of Ramadi last Sunday has sparked questions about the effectiveness of the Obama administration's approach in Iraq. That strategy mixes retraining and rebuilding the Iraqi army, prodding Baghdad to reconcile with the nation's Sunnis and bombing Islamic State group targets from the air without committing American ground combat troops.
Mr Carter defended the use of US air strikes as an effective part of the fight against the Islamic State group but said they are not a replacement for Iraqi forces defending their country.
"We can participate in the defeat of ISIL," he said, using another acronym for Islamic State. "But we can't make Iraq ... a decent place for people to live - we can't sustain the victory, only the Iraqis can do that. And, in particular in this case, the Sunni tribes to the west."
The Pentagon estimated that when Iraqi troops abandoned Ramadi, they left behind half a dozen tanks, a similar number of artillery pieces, a larger number of armoured personnel carriers and about 100 wheeled vehicles like Humvees.
Over the past year, defeated Iraq security forces have repeatedly left behind US-supplied military equipment, which the US has targeted in subsequent air strikes against Islamic State forces.