Fighters quickly withdrew without putting up much resistance, leaving behind several tanks

The Islamic State group's takeover of the Iraqi provincial capital, Ramadi, has prompted criticism from US Defense Secretary Ash Carter and raised new questions about the Obama administration's strategy to fight the extremist group.

The group, which had already seized a strategically important area of the Middle East, seized Ramadi in central Iraq a week ago.

The Obama administration's approach in Iraq is a blend of retraining and rebuilding the Iraqi army, prodding the Shiite-dominated government in Baghdad to reconcile with the nation's Sunnis and attacking IS targets from the air without committing US ground combat troops.

US President Barack Obama's strategy is predicated on Baghdad granting political concessions to the country's alienated Sunnis, who are a source of personnel and money for extremists. However, Baghdad has continued to work closely with Shiite militias backed by Iran, which have been accused of atrocities against Sunnis, a religious minority in Iraq that ruled until Saddam Hussein fell from power.

The US has sought to reach out on its own to Sunni tribes and is training some Sunni fighters, but those efforts have been limited by the small number of US troops on the ground.

Carter said in an interview aired on Sunday that Shiite-led Iraqi forces did not show a "will to fight" in the battle for Ramadi, a Sunni city.

Although Iraqi soldiers "vastly outnumbered" their opposition in the capital of Anbar province, the Sunni heartland, they quickly withdrew from the city without putting up much resistance, Carter said on CNN's State of the Union.

The Iraqis left behind large numbers of US-supplied vehicles, including several tanks, now presumed to be in Islamic State hands.

"What apparently happened is the Iraqi forces just showed no will to fight," Carter said. "They were not outnumbered; 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."

Iraqi Prime Minister Haider al-Abadi expressed disbelief at Carter's stinging comments.

"I'm surprised why he said that. I mean, he was very supportive of Iraq. I am sure he was fed with the wrong information," Abadi told the BBC.

Ahmed al-Assadi, spokesman of the Hashed al-Shaabi umbrella organization for Shiite militia and volunteers called in after Ramadi's fall, reacted angrily to Carter's comments.

"This lack of will the US defense secretary mentioned is how the enemies of Iraq have tried to depict the Iraqi security forces," he said.

The government has nonetheless admitted to shortcomings, vowing to investigate the chaotic retreat from Ramadi and punish "recalcitrant" fighters.

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

IS jihadists in different parts of Syria's Homs province have executed 67 civilians and 150 members of regime forces since May 16, according to the Syrian Observatory for Human Rights.

Observatory director Rami Abdel Rahman said whole families had been executed, including children with their parents, after Syrian state media said IS had carried out a "massacre" in Palmyra, slaughtering some 400 civilians.

AP - AFP