Iraqis from Ramadi flee toward Baghdad. (Hadi Mizban/AP)

Iranian-allied ­Shiite militias mobilized fighters Monday to try to wrest back the capital of Iraq’s largest province from Islamic State fighters, raising the specter of a major clash that could inflame the country’s sectarian relations.

Thousands of militiamen, many of them veterans of the anti-American insurgency a decade ago, are expected to join Iraqi government forces in trying to push Islamic State fighters out of Ramadi.

U.S. and Iraqi government officials have been wary about the use of the Shiite militias in Anbar province, the heartland of Iraq’s Sunni minority, fearing it could fuel sectarian strife. But with Iraq’s undersupplied and poorly managed security forces driven from Ramadi over the weekend, Prime Minister Haider al-Abadi called Sunday night for the militias to join the battle.

Anbar’s provincial council voted in favor of requesting support from the militias, an extraordinary move that the lawmakers said was necessary in light of the collapse of pro-government ­forces.

Analysts warned, however, that the move was fraught with risk. Islamic State fighters, who are Sunni, may use the presence of Shiite fighters to paint the conflict as a sectarian war and increase their support among local residents. The militias have already been accused by human rights groups of revenge attacks against Sunni citizens in areas retaken from the Islamic State.

“The Iraqis are resorting to the forces that the Americans and the international community don’t want them to use: the Shiite militias. This will intensify Sunni-Shiite conflict in Iraq and the region,” said Salman Shaikh, director of the Brookings Doha Center.

On Monday, Islamic State fighters tried to push closer toward Baghdad, 80 miles to the east of Ramadi.

Battles flared on the outskirts of the village of Husaybah, about eight miles east of Ramadi and close to the Euphrates River, said Rafia al-Fahdawi, a tribal elder from the village who spoke by telephone from Baghdad.

About 10 village defenders have been killed in the attacks on Husaybah, he said. The militants’ wider aim, however, appeared to be clearing a path for an assault on the Habbaniyah air base, about 20 miles east of Ramadi on the road to Baghdad.

U.S.-led airstrikes appeared to escalate in the area in possible attempts to slow further gains by the Islamic State. At least 19 airstrikes have been conducted near Ramadi since Friday, according to the American-directed military coalition targeting the Islamic State in Iraq and Syria.

Gen. Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, said in a statement that the Islamic State’s advance into ­Ramadi was “a serious setback for its long-suffering inhabitants. It is also a setback for the ISF,” or Iraqi Security Forces.

The Islamic State now controls Ramadi, a major city in Iraq. Here's why that matters and what to expect next. (Alice Li/The Washington Post)

“We will continue to support Iraq’s Security Forces with U.S. airstrikes, training, and equipment,” he said.

A Pentagon spokesman, Col. Steve Warren, said in Washington that Shiite militiamen are likely to play a role in the battle for Ramadi. “As long as they are controlled by the central Iraqi government, then [the militias] will participate,” he said.

The Iraqi government relied heavily on Shiite militias to take back the city of Tikrit, in Salahuddin province, from Islamic State fighters last month. But using them in Anbar may be more controversial and difficult. The huge western province was a hotbed of resistance to the U.S.-led invasion in 2003 and became home to the country’s al-Qaeda affiliate. The province is now largely controlled by the Islamic State.

The U.S.-backed Iraqi government had hoped to arm and train local Sunnis in Anbar to fight the Islamic State, just as Sunni tribesmen in the province were persuaded to turn on al-Qaeda in Iraq a decade ago, in a major triumph for U.S. military forces. But the newer program never got off the ground.

Police and military forces in Ramadi had held off Islamic State forces for months before the attack launched Thursday evening. The insurgents killed more than 500 people in Ramadi during the three-day offensive, according to Anbar’s provincial council members. Iraqi officials, police and residents accused the extremists of summarily executing police, tribal fighters and civilians, including women and children. Close to 25,000 people have fled Ramadi in the past few days, according to U.N. officials.

Charles Lister, a Middle East expert and author of “Profiling the Islamic State,” a paper written for the Brookings Doha Center, said the Shiite militias could provide much-needed muscle in confronting the extremist group. But there are key questions over the role that the militias will play, he said, such as whether they will lead the fight and what they will do with areas seized from the Islamic State.

“My fear is that if these militias play a long-term and preeminent role, this will just give the extremists an opportunity to push the sectarian narrative that everyone is trying to prevent,” Lister said. He described that as a “worst nightmare” scenario for U.S. officials.

The Iraqi government’s reliance on the militias also potentially gives Iran extra leverage in the country. In a sign of Iran’s increasing role, the Iranian defense minister, Brig. Gen. Hossein Dehqan, arrived for talks Monday with authorities in Baghdad.

Washington faces a complicated decision over how much to support the Iranian-aided militiamen — who battled U.S. troops after the 2003 invasion.

One of the most powerful ­Shiite militias, Asaib Ahl al-Haq, planned to send 1,500 fighters from its special forces unit to Anbar on Tuesday, said spokesman Naeem al-Aboudi. The militiamen, he said, had also fought to support President Bashar al-Assad in Syria’s civil war and had battled U.S. troops in Iraq.

“They are the best,” he said of the force.

Another major Shiite faction, Kitaeb Hezbollah, has begun sending some of its most elite fighters to Anbar, said spokesman Jafar al-Husseini.

“These are well-trained forces who have experience in Syria, and they fought the Americans,” he said.

Missy Ryan in Washington contributed to this report.

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