ANBAR PROVINCE, Iraq -- Thousands of Shi'ite militia fighters have arrived at a military base near Ramadi in preparation for what they say will be a counteroffensive to retake the western Iraqi city seized by Islamic State (IS) militants on May 17.
Prime Minister Haidar al-Abadi had been previously reluctant to deploy Shi'ite militia in Sunni-dominated Ramadi, capital of Anbar Province.
But Abadi on May 17 ordered fighters from a Shi'ite militia umbrella group known as Hashid al-Shaabi (Popular Mobilization Units) to join the Ramadi front after Anbar's provincial council and Sunni tribal leaders in the province pleaded for help.
Sabah Karhout, the head of the Anbar Provincial Council, told RFE/RL that council members had all voted for the deployment of Shi'ite fighters in Sunni areas after government forces withdrew from their bases in Ramadi on May 17.
Meanwhile, Islamic State militants searched door-to-door for policemen and pro-government fighters and threw bodies in the Euphrates River in a bloody purge May 18.
Residents, speaking on condition of anonymity because they feared reprisals, said the IS militants had 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.
Some 500 civilians and soldiers have died in the extremist killing spree since the final push for Ramadi began May 15, authorities said.
RFE/RL's correspondent in Anbar Province reported that government forces that pulled out of Ramadi on May 17 regrouped overnight and launched counterattacks from the western side of the provincial capital early on May 18.
He said the attacks were being supported by an increased number of U.S.-led coalition air strikes as well as attacks by Iraqi aircraft, including helicopter gunships.
The White House said on May 18 the loss of Ramadi was a "setback" but the United States and its coalition will help Iraqi forces retake the city.
"There's no denying that this is indeed a setback, but there's also no denying that we'll help the Iraqis take back Ramadi," White House spokesman Eric Schultz told reporters traveling with President Barack Obama.
The United Nations said on May 18 that up to 25,000 people fled Ramadi after it was attacked by IS militants and most of them headed toward Baghdad.
RFE/RL's correspondent also reported that the IS militants continued to hold villages around the provincial capital on May 18, as well as some positions in western neighborhoods of the city itself.
Meanwhile, IS militants and witnesses said the Sunni-led extremists advanced eastward in armored vehicles on May 18 from Ramadi to the outskirts of the town of Al-Khalidiyah, near Habbaniyah Lake and the Habbaniyah base where the Shi'ite fighters were massing.
RFE/RL's correspondent reported that convoys of government military vehicles and troop reinforcements were moving west from Baghdad toward Ramadi to take part in the ongoing battle.
Militias Gather Forces
Karhout said on May 18 that the Shi'ite fighters at the Habbaniya base were "now on standby" about 20 kilometers from Ramadi.
The Anbar Provincial Council said some 3,000 Shi'ite fighters would take part in "the liberation of all Ramadi areas in which IS militants took positions."
Yusif al-Kilabi, the security spokesman for the Popular Mobilization Units, confirmed in an interview with RFE/RL on May 18 that "thousands" of Shi'ite fighters were being sent to the Ramadi front, "enough to liberate the city" from IS militants.
Hadi al-Ameri, leader of the Badr militia fighters within the Popular Mobilization umbrella group, said on May 18 that Anbar's Sunni leaders should have taken up his offer of military support sooner.
Ameri said he "holds the political representatives of Anbar responsible for the fall of Ramadi because they objected to the participation of Hashid al-Shaabi in the defense of their own people."
But he said his forces were still prepared to join the battle against the IS militants at Ramadi in the days ahead.
A spokesman for Kataeb Hizballah, another leading Shi'ite paramilitary group within the Popular Mobilization Units, said his organization had fighters ready to join the Ramadi front from three directions.
Spokesman Jaafar al-Husseini said on May 18 that the Kataeb Hizballah reinforcements could start advancing on Ramadi within 24 hours.
Other Shi'ite militia groups announced they also already had units in Anbar Province -- including around the cities of Fallujah and Habbaniyah -- that were ready to close in on Ramadi.
WATCH: Many Iraqis who have fled the IS advance in Anbar Province have found refuge at a former holiday camp by Lake Habbaniya, just south of Ramadi.
Iranian Military Cooperation
In Baghdad, Iranian Defense Minister Hossein Dehghan was meeting on May 18 with Abadi and President Faud Masum, as well as the Iraqi ministers of defense, foreign affairs, and the interior.
IRNA reported that Dehghan was in Baghdad to "explore ways to expand defense and military cooperation and promote cooperation against terrorism to create a secure and stable region."
Although Iranian generals have been reported in Iraq in the past, working with the Iraqi Shi'ite militia, Dehghan's May 18 visit to the Iraqi capital marked an unprecedented level of official military cooperation between Baghdad and Tehran.
On May 17, Abadi met with the head of the U.S. Central Command, General Lloyd Austin, to discuss the latest developments in the fight against IS militants -- including the decision to bring the Iranian-backed Shi'ite militia into the battle for Ramadi.
Washington has also been reluctant in the past to deploy Shi'ite militias in Sunni areas of Anbar Province.
Instead, the United States had advocated the development of locally recruited Sunni forces to combat the Sunni-led IS extremists in the area.
The Popular Mobilization Units played a critical role in the government offensive to recaptured Tikrit from IS militants to the north of Baghdad in April.
But some Shi'ite fighters were accused of abuses against Sunni civilians in Tikrit as revenge for mass killings and other atrocities against Shi'a when the IS extremists seized large swaths of territory across northern and western Iraq in 2014.
With reporting by RFE/RL's Radio Free Iraq correspondent Samira Ali Mandi in Prague, Reuters, AP, AFP, and BBC