The takeover of the Iraqi city of Ramadi by Islamic State (Isis) fighters is a "significant setback" and makes the coalition battle plan against the insurgents "longer and less pretty", an expert said on Monday (18 May).

"I think it's really quite bad on multiple fronts," national security and defence policy analyst Michael O'Hanlon of the Brookings Institution in Washington said.

"It doesn't mean that over time ISIL (Isis) is going to take Baghdad or control the country, and I still think over time ISIL will be defeated, but this is fairly bad news and it also sets back the clock on what we had hoped would be by now a preparation for a counterattack against Mosul, the largest and most important city that ISIL holds," O'Hanlon said.

Isis militants said they had taken full control of the western Iraqi city of Ramadi on Sunday (17 May) in the biggest defeat for the Baghdad government since last summer.

US Secretary of State John Kerry along with the Pentagon played down the development on Monday and expressed confidence that the takeover would be reversed in coming weeks.

"I'm not quite as inclined to talk this one down as some people," O'Hanlon said.

"I think it does mean that the overall battle plan against ISIL is going to have to be slower than we predicted," he said, adding: "And I think what it means is that it's not to say there's no hope. But it says it's going to take longer and be less pretty."

The fall of the city called into question the relative strength of Iraq's army after months of US-led backing and air strikes.

"We're going to have to work with some of these Shi'ite militias which means finding some way to regulate them so that the relatively more controllable ones can be worked into the battle plans. Otherwise, we're just going to have to see this improvising which raises more questions than it answers, and raises a lot of fears among the Sunni," O'Hanlon said.

When asked if the death of Islamic State leader Abu Sayyaf during a daring US raid into Syria would substantially affect the insurgent's operations, O'Hanlon said it was too soon to tell.

"In the history or terrorist organizations or large extremist movements of this type, usually what you see is that if the group is reasonably well established, the loss of a relatively high level person does not radically set them back," he said.

"We're going to have to watch and see. This could be a person who was just so talented or so capable that he really can't be easily replaced."