BAGHDAD -- The capital of Anbar province fell to Islamic State militants Sunday as hundreds of police personnel, soldiers and tribal fighters abandoned the city, prompting the Iraqi premier to order Iranian-aligned Shia militias to join the fight to win back control.

The fall of Ramadi represented a huge victory for ISIS, an acronym for the Islamic State militant group, and dealt a profound blow to Iraq's U.S.-backed government, led by Prime Minister Haider al-Abadi, and its campaign to drive the extremist group out of the war-torn nation.

Just 24 hours before, officials in Baghdad announced that military reinforcements had been dispatched to defend Ramadi, capital of Iraq's largest province, against a brutal assault that began on Thursday. But by Sunday, even the roads to the Iraqi capital of Baghdad, 80 miles to the east, appeared vulnerable to the militant advance.

"Ramadi has fallen," Muhannad Haimour, a spokesman for the Anbar governor, said Sunday. "The city was completely taken . . . It was a gradual deterioration. The military is fleeing." Early Monday, Haimour said about 500 people -- both civilians and Iraqi soldiers -- have been killed and about 8,000 people have fled over the past few days as Ramadi fell to ISIS.

Yesterday's developments, including al-Abadi's decision to deploy Shia militias to the country's Sunni heartland, could complicate the U.S.-led campaign, which in recent days has included U.S. airstrikes against militant positions in Ramadi in an effort to keep the city in government hands. U.S. officials have expressed concern over the divisive, sectarian-motivated Iranian-backed Shia militias.

U.S. Secretary of State John Kerry early Monday said he remained confident about the fight against the Islamic State group, despite the setbacks like the loss of Ramadi. Kerry, traveling through South Korea, said that he's 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 yesterday it conducted seven airstrikes in Ramadi in the past 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."

On state television Sunday, al-Abadi's announcement, which contained few details, also included a plea for pro-government forces not to abandon their positions in Anbar.

Security forces retreated from the Malaab area of Ramadi at 1:30 p.m., abandoning about 60 military vehicles including military-grade Humvees to ISIS, said Col. Nasser al-Alwani of the Ramadi police. About half of the abandoned vehicles were sent by the U.S.-backed government on Saturday to reinforce the neighborhood, he added.

Along with soldiers and counterterrorism units, the force of about 400 police officers under his command retreated in their vehicles to the east, al-Alwani said. ISIS fighters besieged them on all roads, forcing them to abandon the vehicles.

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In a sign of the seriousness over the Anbar situation, al-Abadi's order to Shia militias came just hours after Anbar's provincial council voted in favor of allowing the irregular forces, known by the government as "popular mobilization units," to participate in the battle to retake the city. With AP