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.
"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.
Also Read
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.
The coalition said it conducted another 17 strikes over a 24-hour period straddling Saturday and Sunday, including seven in Anbar.
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 100 kilometres west of Baghdad, was reminiscent of the complete collapse of federal forces in Mosul a year ago, when jihadists took the country's second city almost without a fight.