Islamic State fighters wrested control of Ramadi on Sunday as Iraqi troops beat a hasty retreat from the strategic city, according to government and opposition accounts.
A spokesman for the governor of Anbar province confirmed that Ramadi, the provincial capital, had fallen to the militants, though he said there were still "pockets of resistance."
The takeover appeared to be a major victory for Islamic State, which has withstood a large-scale U.S. bombing campaign and still maintains strongholds in both Iraq and neighboring Syria.
Supporters of the militant group uploaded dozens of images on social media depicting smiling fighters standing before what they said were landmarks in Ramadi, about 60 miles west of Baghdad. Some images also showed what the militants said were police and army Humvees and other armored vehicles fleeing Ramadi before Islamic State's advance.
A statement released by Islamic State said the city had been completely overrun after its forces drove out the Iraqi army's 8th Brigade and Anbar operations command, "which led to the killing of tens [of government soldiers] and the fleeing of hundreds of the apostates."
Islamic State, the dominant faction in the Sunni Muslim-led insurgency in Iraq, considers the Shiite Muslim-dominated government as an "apostate" entity whose people should be killed.
The statement also asserted that Islamic State netted a number of tanks and rocket launchers.
Muhannad Haimour, the spokesman for Anbar's governor, said in a telephone interview Sunday that although the "city had officially fallen into Daesh's hands, there are still pockets of resistance and fighting is ongoing in some neighborhoods." Daesh is the Arab acronym for Islamic State.
Ramadi's fall was not unexpected, the spokesman said. For months, the government had held out in the city center against the militants, but the tide of battle had turned in recent days.
"The city had been fighting for a very long time and unfortunately there was never enough help for the city," said Haimour, explaining that pro-government Sunni tribal fighters had been clamoring for assistance and reinforcements from Baghdad to little effect.
The spokesman credited Islamic State's victory to more aggressive tactics adopted by the group, including the extensive use of large, explosives-packed suicide vehicles.
"Daesh started ... armoring bulldozers and heavy construction equipment driven by suicide bombers and gained momentum," the spokesman said.
Anbar, encompassing a full third of Iraq and sharing borders with Jordan, Syria and Saudi Arabia, has been a stronghold for the Sunni insurgency since the U.S.-led invasion of Iraq in 2003 that ousted Saddam Hussein, who was popular in the Sunni-dominated province.
The government's defeat in Anbar highlights how sectarian calculations continue to dominate Iraq in the post-Hussein era. Residents of Sunni-dominated areas such as Anbar province and the northern city of Mosul, also under Islamic State control, have little confidence in the central government.
Many Sunni residents and tribal leaders view Islamic State as a better alternative despite the militants' brutal ruling style. Many local Sunnis from Ramadi and elsewhere in Anbar have joined Islamic State.
The central government has hesitated to deploy Shiite-led militias in Ramadi for fear of sectarian clashes with the province's Sunni residents. The militias, known as Popular Mobilization forces and some backed by Shiite Iran, are considered the best pro-government fighting force.
On Sunday, however, Iraqi Prime Minister Haidar Abadi directed "the commission of the Popular Mobilization to prepare along with the armed forces and sons of the [Sunni] tribes to liberate Anbar from Daesh," according to Al Iraqiya state news broadcaster.
The Shiite militias were key in the retaking in March of the largely Sunni city of Tikrit, which had long been under Islamic State control. There have since been accusations of abuses by the militias in Tikrit.
Many U.S. troops lost their lives or were wounded fighting against Sunni militants in Ramadi during the U.S. occupation of Iraq, which ended in 2011. The militants also control nearby Fallouja, another place where the U.S. military launched large offensives to oust Sunni militants.
The latest collapse of Iraqi government forces came despite numerous airstrikes on militant positions around Ramadi from the U.S.-led coalition. The setback underscores the limitations of airstrikes that lack an effective military partner on the ground.
The fight for Ramadi has caused thousands of civilians to flee toward Baghdad, but many are not allowed into the capital because they are suspected of being Islamic State sympathizers.
In a news conference Sunday in Baghdad, Anbar Gov. Suhaib Rawi demanded the officials facilitate the entry into the capital of the displaced people of Ramadi and give assistance in this "unprecedented humanitarian crisis" for Anbar, according to local broadcaster Sumariya News.
Bulos is a special correspondent.