CONTESTED IRAQI CITY OF RAMADI FALLS TO ISLAMIC STATE GROUP,

BAGHDAD—The contested city of Ramadi fell to the Islamic State group on Sunday, as Iraqi forces abandoned their weapons and armored vehicles to flee the provincial capital in a major loss despite intensified U.S.-led airstrikes.

Bodies, some burned, littered the streets as local officials reported the militants carried out mass killings of Iraqi security forces and civilians. Online video showed Humvees, trucks and other equipment speeding out of Ramadi, with soldiers gripping onto their sides.

“Ramadi has fallen,” said Muhannad Haimour, a spokesman for the governor of Anbar province. “The city was completely taken. … The military is fleeing.”

With defeat looming, Prime Minister Haider al-Abadi ordered security forces not to abandon their posts across Anbar province, apparently fearing the extremists could capture the entire desert region that saw intense fighting after the 2003 U.S.-led invasion to topple dictator Saddam Hussein.

Sunday’s retreat recalled the collapse of Iraqi security forces last summer in the face of the Islamic State group’s blitz into Iraq that saw it capture a third of the country, where it has declared a caliphate, or Islamic State. It also calls into question the Obama administration’s hopes of relying solely on airstrikes to support the Iraqi forces in expelling the extremists.

Earlier Sunday, al-Abadi ordered Shiite militias to prepare to go into the Sunni-dominated province, ignoring U.S. concerns their presence could spark sectarian bloodshed. By late Sunday, a large number of Shiite militiamen had arrived at a military base near Ramadi, apparently to participate in a possible counter-offensive, said the head of the Anbar provincial council, Sabah Karhout.

“We welcome any group, including Shiite militias, to come and help us in liberating the city from the militants. What happened today is a big loss caused by lack of good planning by the military,” a Sunni tribal leader, Naeem al-Gauoud, told the Associated Press.

He said many tribal fighters died trying to defend the city, and bodies, some charred, were strewn in the streets, while others had been thrown in the Euphrates River. Ramadi mayor Dalaf al-Kubaisi said that more than 250 civilians and security forces were killed over the past two days, including dozens of police and other government supporters shot dead in the streets or their homes, along with their wives, children and other family members.

The U.S.-led coalition said Sunday it conducted seven airstrikes in Ramadi in the last 24 hours. “It is a fluid and contested battlefield,” said Army Col. Steve Warren, a Pentagon spokesman. “We are supporting (the Iraqis) with air power.”

The final push by the extremists began early Sunday with four nearly simultaneous bombings that targeted police officers defending the Malaab district in southern Ramadi, a pocket of the city still under Iraqi government control, killing at least 10 police and wounding 15, authorities said. Among the dead was Col. Muthana al-Jabri, the chief of the Malaab police station, they said.

Later, three suicide bombers drove their explosive-laden cars into the gate of the Anbar Operation Command, the military headquarters for the province, killing at least five soldiers and wounding 12, authorities said.