Iraqi forces closed in on Ramadi Tuesday and launched an operation aimed at cutting off the jihadists in Anbar province before a major offensive to retake the city.
Ten days after the Islamic State group's shock capture of the capital of Iraq's largest province, a spokesman said the latest operation was only a preparatory move before an assault on Ramadi.
The operation will see a mix of security forces and paramilitaries move south towards the city from Salaheddin province, said Hashed al-Shaabi spokesman Ahmed al-Assadi.
The Hashed al-Shaabi ("popular mobilisation" in Arabic) is an umbrella group for mostly Shiite militia and volunteers, which the government called in after the Islamic State group captured Ramadi on May 17.
"The operation's goal is to liberate those regions between Salaheddin and Anbar and try to isolate the province of Anbar," Assadi told AFP.
He said it had been dubbed "Operation Labaik ya Hussein", which roughly translates as "We are at your service, Hussein" and refers to one of the most revered imams in Shiite Islam.
The Hashed said 4,000 men were heading to the northern edge of Ramadi.
Prime Minister Haider al-Abadi and his US allies had been reluctant to deploy Iran-backed Shiite militia in Anbar, a predominantly Sunni province.
Anbar's provincial capital Ramadi had resisted IS assaults for more than a year but fell earlier this month after a massive jihadist offensive and a chaotic retreat by security forces.
The Islamic State group controls most of Anbar, a huge province which borders territory also under its control in neighbouring Syria.
Pockets of government control include some eastern areas near the capital, the city of Haditha, parts of the town of Al-Baghdadi and the Al-Asad air base, where hundreds of US military advisers are stationed.
Regular forces and Hashed al-Shaabi paramilitaries also made progress south and west of Ramadi, an army lieutenant colonel told AFP, and retook an area called Al-Taesh.
"The Iraqi security forces and Hashed al-Shaabi have now cut off all supply routes for IS in Ramadi from the south," provincial council member Arkan Khalaf al-Tarmuz said.
Over the past week, IS is likely to have built up its defences by rigging much of Ramadi with explosives, the jihadist group's weapon of choice.
Washington on Monday moved to appease Baghdad after Iraq's leadership reacted angrily to comments by the Pentagon chief accusing Iraqi forces of "lacking the will to fight".
Ashton Carter's remarks to the CNN news channel were widely perceived as unfair in Iraq, where some forces have put up valiant resistance to IS assaults.
In a call to Abadi, the White House quoted Vice President Joe Biden as saying he "recognised the enormous sacrifice and bravery of Iraqi forces over the past 18 months in Ramadi and elsewhere."
Tehran, the main backer of the paramilitary groups that were sent to Anbar's rescue, was gloating and suggested it was Washington that was indecisive in its approach to IS.
"How can you be in that country under the pretext of protecting the Iraqis and do nothing? This is no more than being an accomplice in a plot," said General Qassem Suleimani, the Revolutionary Guards' commander of foreign operations.
The US-led coalition has carried more than 3,000 strikes against IS targets in Iraq and Syria over the past 10 months.
Baghdad and Washington had boasted that IS was a waning force after months of territorial losses but the fall of Ramadi signalled that the jihadist group may have been written off too soon.
Its seizure of the city prompted 55,000 residents to flee their homes, according to the United Nations.
Many of them have been prevented from crossing into other provinces, for fear they have been infiltrated by IS fighters.
Some Sunni Arab politicians and activists have described the move as unconstitutional and discriminatory against the minority community.
The International Rescue Committee said the restriction was forcing some people to return to conflict areas.
"Thousands of people fleeing Ramadi are stuck at checkpoints or being denied entry to safe areas," IRC's Syrian crisis response regional director, Mark Schnellbaecher, said.
"For some people the situation has become so hopeless that they are returning to the conflict in Ramadi."