Fear of a possible Islamic State bloodbath sent tens of thousands of Iraqis fleeing Ramadi on Monday after government forces abandoned the city -- just 80 miles from Baghdad -- in what one U.S. military official conceded was a fight "pretty much over."

Some 25,000 people have fled the embattled streets of Ramadi as thousands of ISIS fighters seized the key Iraqi city, killing some 500, and reportedly going door-to-door looking for Iraqi government troops and police to run out of town. 

“There have been executions in the streets of Ramadi," Muhannad Haimour, a spokesman for the Anbar provincial government, told NBC News Monday. ISIS extremists used vehicles, bulldozers rigged with explosives and suicide bombers to overrun the city after weeks of battles in the street.  

"The situation in the city is absolutely terrible," Haimour said. "The city is in very bad shape."

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, called ISIS' gains "a serious setback" for both the city's inhabitants and the Iraqi Security Forces.

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"Much effort will now be required to reclaim the city," Dempsey said.

The fight for Ramadi is “pretty much over for now,” a U.S. military official told Fox News, after ISIS overran the beleaguered Iraqi Army to take control of the city Sunday. 

Iraqi security forces abandoned their Anbar Operations Center in Ramadi overnight, leaving the city almost completely in ISIS control, according to the U.S. official, who has seen the latest intelligence reports from Ramadi.

Although there were a large number of Iraqi security forces occupying Ramadi, most troops fled after ISIS fighters began their assault on the city center Sunday, leaving behind Humvees and armored vehicles supplied by the U.S. military, a separate senior U.S. military official told Fox News.

"The Iraqi security forces were pushed out by a much smaller [ISIS] force," the official said.

The takeover followed a three-day siege that began with a wave of ISIS car bombs and which dealt a devastating blow to the Baghdad government and the U.S. forces providing logistical support. On Monday, Shia militias converged on the city, some 70 miles west of the capital, in a bid to retake it.

Ramadi's streets were deserted Monday, with few people venturing out of their homes to look for food, according to two residents reached by telephone.

The militants, meanwhile, were storming the homes of policemen and pro-government tribesmen, particularly those from the large Al Bu Alwan tribe, of whom they detained about 30, the residents said. The militants went door-to-door with lists of alleged pro-government collaborators. Homes and stores owned by a pro-government Sunni militia known as the Sahwa were looted or torched.

The residents spoke to The Associated Press on condition of anonymity because they feared reprisals by the militants.

Youssef al-Kilabi, a spokesman for the Shiite militias fighting alongside government forces, told the AP on Monday that the Iranian-backed paramilitary forces have drawn up plans for a Ramadi counter-offensive in cooperation with government forces.

We will "eliminate this barbaric enemy," al-Kilabi vowed. He did not elaborate on the plans or the timing of a counter-offensive.

Iran's Defense Minister Gen. Hossein Dehghan flew to Baghdad on a surprise visit for urgent talks with Iraqi leaders.

The fall of Ramadi was a stunning defeat for Iraq's security forces and military, which fled as the ISIS rebels overwhelmed the last hold-out positions of pro-government forces, despite the support of U.S.-led airstrikes targeting the extremists

The retreat by Iraqi forces was reminiscent of the nation's earliest battles against ISIS, including the fall of Mosul, when poorly trained Iraqi soldiers shrank from the black-clad Islamist army, leaving guns and other gear behind for the terrorists to capture.

In Ramadi Sunday, bodies 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," Haimour, a spokesman for the provincial governor of Anbar, told AP Sunday. "The city was completely taken. ... The military is fleeing."

Since Friday, when the battle for the city entered its final stages, "We estimate that 500 people have been killed, both civilians and military," Haimour said.

The figures could not be independently confirmed, but Islamic State militants have in the past killed hundreds of civilians and soldiers in the aftermath of their major victories.

The Pentagon is aware of reports that Iran-backed Shia militias have been asked by Iraq's Prime Minister to lead the fight to take back Ramadi.  Iran's defense minister arrived in Baghdad today for talks with his counterpart, in what the media is calling an "emergency meeting."

When asked if the U.S. military planned to increase its involvement in the campaign to defeat ISIS, the senior U.S. military official said, "The Iraqis have to want it more than we want it."

A Sunni tribal leader, Naeem al-Gauoud, said many tribal fighters died trying to defend the city and their bodies 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.

Secretary of State John Kerry, speaking in South Korea, called Ramadi a "target of opportunity" for extremists, but said he was confident that ISIS' gains could be reversed in the coming days. Kerry also said that he has long said the fight against the militant group would be a long one, and that it would be tough in the Anbar province of western Iraq where Iraqi security forces are not built up.

The U.S.-led coalition said Sunday it had 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."

Iraqi Prime Minister Haider al-Abadi ordered security forces not to abandon their posts across Anbar province, apparently fearing the extremists could capture the entirety of the vast Sunni province that saw intense fighting after the 2003 U.S.-led invasion of the country 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.

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.

On a militant website frequented by ISIS members, a message from the group claimed its fighters held the 8th Brigade army base, as well as tanks and missile launchers left behind by fleeing soldiers. The message could not be independently verified by the AP, but it was similar to others released by the group and was spread online by known supporters of the extremists.

Backed by the U.S.-led airstrikes, Iraqi forces and Kurdish fighters have made gains against ISIS, including capturing the northern city of Tikrit. But progress has been slow in Anbar, a Sunni province where anger at the Shiite-led government runs deep and where U.S. forces struggled for years to beat back a potent insurgency. American soldiers fought some of their bloodiest battles since Vietnam on the streets of Ramadi and Fallujah.

Fox News' Lucas Tomlinson and the Associated Press contributed to this report.