Iraq's leadership was indignant after US accusations that its forces lack the will to fight but assessments of the army's ability suggest the rank-and-file have reasons to shy from battle.

Pentagon chief Ashton Carter said Sunday, a week after the loss of Ramadi to the Islamic State group, that there was "an issue with the will of the Iraqis to fight".

The fall of the capital of Iraq's largest province to the jihadists saw a chaotic retreat of security forces and dealt Baghdad its worst blow in almost a year.

Carter's words were criticised in Iraq as insulting and divisive and US Vice President Joe Biden later had to call Prime Minister Haider al-Abadi to acknowledge "the enormous sacrifice and bravery of Iraqi forces over the past 18 months".

The problem is not whether Iraqi security forces are valiant enough but just about everything else, analysts said.

"The Iraqi forces that left Ramadi this weekend did so under great pressure and only as a last resort," said the Soufan Group intelligence consultancy in a recent brief.

"It is not bravery the forces lacked but weapons and leadership and rapid logistics," it said.

- Faith vs. money -

Ahmed al-Fadhily wanted to fight IS when it captured swathes of Iraq last year and imposed a reign of terror. But to do so, he quit the army and joined a Shiite militia.

"When I was in the military, we never got ammunition when we asked for it. Sometimes we would be marooned on a base with no supplies," he said.

"We knew nobody would make any sacrifice to rescue us. It's different in the Hashed al-Shaabi... The fighters have faith," said Fadhily, who now fights for Ketaeb Hezbollah.

The Hashed al-Shaabi ('popular mobilisation' in Arabic) is an umbrella for many Iran-backed Shiite militias and volunteers which is only nominally under the prime minister's control.

"We are fighting to protect our land from Daesh (IS), while most soldiers and officers work for the salaries," said Fadhily.

Yet corruption is still rampant in the security forces despite recent government measures, and low oil prices and the high cost of war are emptying Iraq's coffers fast.

That means some fighters in the security forces have not been paid for months while some corrupt top officers get rich, policemen and soldiers say.

Iraq is still paying the price of the de-Baathification campaign that followed the US-led toppling of Saddam Hussein in 2003, with many of the former Sunni regime's best-trained military commanders defecting to the jihadists.

The US-led coalition fighting IS sent thousands of military advisers and trainers to Iraq last year.

But transforming what was still recently derided as a million-strong "checkpoint army" into a lean, efficient and professional military cannot be achieved overnight, or even in months, analysts say.

The much-vaunted US-trained Golden Division has stood out since IS seized swathes of the country.

It successfully spearheaded the fightback on several fronts but it has been overused and, according to several reports, it retreated just the like the rest in Ramadi.

- Weak regime, weak army -

IS's hyper-aggressive tactics -- including the use of terrifying, armoured suicide truck bombs -- have played no small part in the Iraqi army's modest showing.

Ahmed Ali, an analyst with the Education for Peace in Iraq Centre, argued that Carter's remarks were inopportune but admitted that the list of woes afflicting the army was long.

"These issues include logistics challenges, poor command and control, lack of unit coordination, intelligence deficit, and difficulty securing supply lines," he said.

In Ramadi, some forces suddenly found themselves surrounded by IS as panicked units pulled out without alerting others, some fighters told AFP.

Fadhily recalled the lack of trust he had for his officers when he was in the army.

"Lots of them were corrupt and this meant we had no confidence in their decisions in the battlefield," he said.

Sarmad al-Taei, a columnist for Iraq's independent and secular Al-Mada newspaper, said the army's performance was a reflection of the political leadership's weakness.

"No-one has confidence in the political administration, not its opponents and not even its supporters," he said.

He compared the army to the public sector -- plethoric and ineffective.

"The Hashed are like the private sector, which is always better," Taei said of the Shiite-dominated force, which was formed after Iraq's regular army collapsed completely in the face of a sweeping IS offensive last year.

"And the Hashed are backed by Iran, a strong and organised state... Dogmatic armies have always been a natural substitute for weak, defeated armies," he said.