Washington has accused Iraqi forces of lacking the will to fight the Islamic State group, which scored a resounding victory a week earlier with the capture of Ramadi. The jihadists had appeared on the back foot in Iraq in recent months but twin offensives on Ramadi and on the ancient Syrian city of Palmyra have swung the momentum.

The loss of Ramadi, capital of Iraq's largest province of Anbar, raised questions over the strategy adopted not only by Baghdad but also by Washington to tackle IS.

Pentagon chief Ashton Carter told CNN the fall of Ramadi, Baghdad's worst military defeat in almost a year, could have been avoided.

"What apparently happened was the Iraqi forces showed no will to fight. They were not outnumbered, and they vastly outnumbered the opposing force, and they failed to fight and withdrew from the site," he said.

"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," he said, using an alternative name for the group.

The US-led coalition air war that began two months after IS seized swathes of Iraq in June 2014 has led to more than 3,000 strikes.

"Air strikes are effective but neither they, or really anything we do, can substitute for the Iraqi forces' will to fight," Carter said.

Several units of Iraqi security forces, including elite troops, defied their chain of command and retreated from Ramadi when IS advanced.

The Anbar police chief has already been replaced and Prime Minister Haider al-Abadi promised an investigation.

The fall of Ramadi, which lies about 100km west of Baghdad, was reminiscent of Mosul a year ago, when jihadists took Iraq's second city almost without a fight.

The capture of the Anbar capital together with the IS takeover of Palmyra in eastern Syria has consolidated the jihadists' grip on the heart of their self-proclaimed caliphate.