BAGHDAD -- Islamic State of Iraq and Syria (ISIS) militants searched door-to-door for policemen and pro-government fighters and threw bodies in the Euphrates River in a bloody purge Monday after capturing the strategic city of Ramadi.
Some 500 civilians and soldiers died in the extremist killing spree since the final push for Ramadi began Friday, authorities said.
As CBS News correspondent Clarissa Ward notes, Ramadi is the capital of the sprawling Anbar province -- a province once secured at the cost of 1,300 American lives. Taking the town was the biggest military victory for ISIS since last summer, when the militants swept Iraq's second-largest city of Mosul.
Responding to a call from Prime Minister Haider al-Abadi, hundreds of Iranian-allied Shiite militiamen rushed to a military base near Ramadi, the capital of overwhelmingly Sunni Anbar province, to prepare for an assault to try to retake the city, Anbar officials said.
The order came despite Obama administration concerns that the presence of Shiite fighters in the Sunni-dominated region could spark sectarian bloodshed. Until now, the defense of Anbar has been in the hands of the Iraqi military fighting alongside Sunni tribesmen, who al-Abadi's Shiite-led government had vowed to arm and support -- something it has done only sporadically.
The Shiite militias have been key to victories against ISIS on other fronts north of Baghdad in recent months. But they have also been widely criticized over accusations of extrajudicial killings of Sunnis, as well as of looting and torching Sunni property -- charges militia leaders deny.
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In the face of ISIS' brutality, some Sunnis appeared ready to accept help even from the Shiite militiamen.
"We welcome any group, including Shiite militias, to come and help us in liberating the city from the militants," said Sunni tribal leader Naeem al-Gauoud, who fought to defend Ramadi and criticized what he called "lack of good planning by the military."
But Abu Ammar, another Anbar native who owns a grocery store in Ramadi, said he saw no difference between ISIS' brutal practices and those of Shiite militiamen.
"If the Shiite militias enter Ramadi, they will do the same things being done by Daesh," he said, using an Arabic acronym for the militant group. "In both cases, we will be either killed or displaced. For us, the militias and IS militants are two faces of the same coin."
The fall of Ramadi prompted Iran's defense minister, Gen. Dossein Dehghan, to make a surprise visit to Baghdad for urgent talks with Iraqi leaders. He met with al-Abadi, who praised Iran's support for Iraq in the face of the militants.
Ward reports that Ramadi fell to ISIS in spite of intensified U.S. efforts to push the militants back, with at least 180 airstrikes in the area over the last month. As the militants flooded Ramadi, U.S.-trained Iraqi security forces appeared to flee. Policemen abandoned their stations, leaving behind their American-supplied weapons and equipment.
Sunday's defeat in Ramadi, 70 miles west of Baghdad, recalled the collapse of Iraqi forces last summer in the face of a blitz by the extremist group, when it took the northern city of Mosul and swept over much of the north and west of the country. Later, ISIS declared a caliphate, or Islamic state, in areas under its control in Iraq and neighboring Syria.
Backed by airstrikes from a U.S.-led coalition since August, Iraqi forces and allied militias have recaptured some of the areas seized by the Islamic State over the past year. But the new defeat in Anbar calls into question the Obama administration's hopes of relying solely on air power to support Iraqi forces in the battle against IS, as well as whether these forces have sufficiently recovered from last year's stunning defeats.
The White House conceded Monday that the loss of Ramadi was "indeed a setback" and reassured Baghdad that it will help Iraqi forces take it back.
The United States, said White House spokesman Eric Schultz, always knew this would be a long fight with ebbs and flows. "Our aircraft are in the air right now and searching for ISIL targets. They will continue to do so until Ramadi is retaken," he said.
Online video showed Humvees, trucks and other equipment speeding out of Ramadi, with soldiers desperate to reach safety gripping onto their sides.
Since Friday, when the battle for the city entered its final stages, an estimated 500 people have been killed, both civilians and military, said a spokesman for the Anbar provincial government, Muhannad Haimour. The figures could not be independently confirmed, but ISIS militants have in the past killed hundreds of civilians and soldiers in the aftermath of their major victories.
Some 8,000 people also fled the city, Haimour said. It was not immediately clear how many people remained in Ramadi - once a city of 850,000 that has been draining population for months amid fighting with the extremists besieging it. An enormous exodus took place in April, when the U.N. estimates some 114,000 residents streamed out of Ramadi and surrounding villages.
On Monday, U.N. deputy spokesman Farhan Haq said more than 6,500 families had fled in recent days, with the hospital in the nearby city of Khaldiya reporting many casualties.
Ramadi's streets were deserted a day after the city's fall, with only a few people venturing out of their homes to search for food, according to residents reached by The Associated Press.
They said ISIS militants were going door-to-door with lists of government sympathizers, and were breaking into the homes of policemen and pro-government tribesmen, particularly those from the large Al Bu Alwan tribe, some 30 of whom had been detained. Homes and stores owned by pro-government Sunni militiamen were looted or torched.
The residents spoke on condition of anonymity because they feared reprisals by the militants.
The militants are now believed to control more than 60 percent of Anbar, which stretches from the western edge of Baghdad all the way to Syria, Jordan and Saudi Arabia. The mostly desert province, where the population is almost entirely from Iraq's Sunni minority, was a heartland of the Sunni insurgency against U.S. troops following the 2003 U.S.-led invasion that toppled dictator Saddam Hussein.