Iraq’s Shiite militias launched an offensive Tuesday intended to put a stranglehold on Islamic State fighters in Ramadi, taking the lead from Iraqi security forces that lost the western city to the extremists just over a week ago.

The operation to cut supply lines and besiege the city from the northeast is “led and managed and planned” by Iraq’s popular mobilization units, a loose formation of Shiite militia groups and volunteers, said Ahmed al-Assadi, a spokesman for the units. There is “coordination and cooperation” with other military forces, he said.

Iraqi Prime Minister Haider al-Abadi initially held back from sending the Shiite militias to Anbar province, of which Ramadi is the capital, because of sensitivities over how the mostly Sunni population would react. Local Sunni tribes, fearing both the Islamic State’s advance and potential militia abuses, had been split on whether the Shiite paramilitaries should join the battle.

However, the fighters were ordered to the province last week after the fall of Ramadi highlighted weaknesses in Iraq’s regular security forces, and the local council requested the assistance. U.S. officials have indicated that they do not object to the units’ involvement in an Anbar offensive as long as they work under the command and control of the Iraqi government.

On a front line in Anbar, where rows of scorched palm trees bore evidence of months of heavy fighting, army soldiers said Monday they had no doubt that the militias were necessary to retake the province.

Iraqi Sunnis fighting Islamic State militants alongside government forces fire their weapons in a battle for Iraq’s Baiji oil refinery on May 25. (Ahmad Al-Rubaye/AFP/Getty Images)

“Of course we can’t fight without the popular mobilization,” said Capt. Hussein Najib, an officer with the Iraqi army’s 11th Division who was manning an artillery unit in Garma, west of Baghdad. “The mobilizations are Iraq’s unity; they are our right hand.”

The units’ religious fervor is needed in the face of the Islamic State’s extremism, added one of his men. “They fight with faith,” 27-year-old Ali Abdulzahra said. “We need their energy.”

In an indication of the religious overtones of Iraq’s war against the Sunni extremists, the offensive to besiege Ramadi has been named “Labayka ya Hussein” — invoking the name of one of Shiite Islam’s most revered figures. A Pentagon spokesman, Col. Steve Warren, described the openly sectarian code name as “unhelpful.”

The operation, launched early Tuesday, aims to secure the remaining areas in neighboring Salahuddin province, where a fierce battle has raged for the oil refinery in Baiji, before moving on to Ramadi, Assadi said.

“It will finish the liberation of Salahuddin and besiege Ramadi, not liberate it yet,” he said. “We expect it will only take a few days, less than a week.”

Assadi said militia forces have received new supplies of “modern weapons” that will be used in the battle and will “surprise the enemy,” but he declined to give further details.

The operation for Ramadi will be led by the “sons” of the city, said Moeen al-Kadhimi, an official with the Badr Organization, one of Iraq’s most prominent Shiite militias. He said he expected that offensive to be completed within the next two weeks.

The United States and Iraq have traded blame for the fall of Ramadi. Iraqi politicians have hit back at comments by Defense Secretary Ashton B. Carter that Iraqis lacked a “will to fight.”

Assadi blamed the United States for failings in the Iraqi military, pointing out the U.S. role in building the force after dismantling Saddam Hussein’s army following the 2003 invasion.

“This is the army that you have trained for eight years,” he said, addressing the U.S. government. “You worked for eight years and made them weak, through policies that were adopted by you. I say that the Iraqi army, supported by the popular mobilizations, do have the will to fight.”

Warren said Tuesday that the withdrawal from Ramadi was caused by low morale in addition to problems with the command structure. Iraqi forces in the city, including elite Golden Division fighters, crumbled in the face of multiple car bombs in a nearly four-day offensive by the Islamist militants.

Mustafa Salim contributed to this report.

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