The Iraqi Defense Ministry says its forces are working with the predominantly Shiite Hashd Al-Shaabi paramilitary force -- also known as the Popular Mobilization Units -- in the effort to retake Ramadi.
The decision to mobilize Shiite forces follows a request for help from Anbar provincial officials, tribal leaders and religious clerics.
"This is a continuation of the Tikrit operation. It is to cut the supply route from Baiji oil refinery in the north to Ramadi. After the call of the Prime Minister, we answered the call," a spokesman for Hashd al-Shaabi said.
He said the group's forces were ready to act and had Ramadi surrounded from three sides.
What's the goal of the Anbar operation?
ISIS forces have been inching east
since capturing Ramadi, seemingly intending to create an ISIS-controlled corridor along the Euphrates River between Ramadi and another Anbar city the militants already held, Falluja. The latter city is just 37 miles (60 kilometers) west of Baghdad.
Standing between Ramadi and Falluja are government-controlled communities like Khalidiya and Habbaniya.
Habbaniya is home to an Iraqi military base, which the Iraqi government now sees as a staging point for Iraqi security forces and Shiite militias hoping to help Sunni tribesmen retake Ramadi and the rest of Anbar.
The Hashd al-Shaabi Shiite militia group said the military operations in Anbar would be in Karna, northeast of Falluja, and around Ramadi.
A major military operation is also underway in Salaheddin province, its media office said, aimed at cutting a key supply route and liberating Baiji city and refinery.
The refinery is considered strategic
because it is key infrastructure and it is on a major approach route to the city of Mosul, which Iraqi forces hope to take back from ISIS in the coming months.
What do we know about the forces ISIS has in Ramadi/Anbar?
According to Iraqi officials, ISIS now controls as much as 80% of Anbar province, CNN's Tim Lister said last week
. He said the group had maintained a presence in the region for nearly two years, first appearing at anti-government protests in Ramadi late in 2013.
The group took over Ramadi after about a year of fighting there.
Lister said ISIS had continued its tactic of stretching Iraqi forces with multiple, simultaneous attacks in different areas -- using great mobility -- to keep the Iraqi forces off balance.
ISIS has relied heavily on vehicle-borne suicide bombings to break down the Iraqi Security Forces' defenses, as well as snipers and street-to-street fighting. Ramadi was a meticulously planned and complex operation, and the willingness of ISIS to take casualties and its merciless treatment of enemies appear to have sapped the morale of Iraqi forces, Lister said.
Why did Ramadi fall?
U.S. Secretary of Defense Ash Carter publicly criticized the Iraqi army
after the defeat at Ramadi, saying it lacked the will to fight, and Deputy Prime Minister and Sunni lawmaker Saleh al-Mutlaq told CNN on Monday
that the withdrawal "surprised all of us."
Iraqi soldier Asa'ad al-Yassiri's army
contingent was among the last to leave the battle. Speaking to CNN,
he bristled at the notion that Iraqi soldiers like him didn't have the will to fight. He faulted the military leadership and logistical failures that left them without adequate resupply and support.
Al-Yassiri said he believed the order to withdraw was a betrayal. The Iraqi government has said it launched an investigation to find out what went wrong and how the order was issued, but so far, no one has given a viable explanation.
What are the chances of success in Ramadi now?
Retired Lt. Gen. Mark Hertling, a CNN military analyst, said he expected that the operation would succeed after "some very difficult fighting."
"My experience with the Iraqi fighters -- with the Iraqi soldiers -- is they are very courageous and they are willing to fight for the three things of God, their tribes and then the nation," he said.
"While there certainly was a 'war of words,' if you will, over the weekend, I think that gave the impetus to have this offensive start a little bit sooner than expected."
The latest operation follows the announcement that an alliance of Iraqi forces opposed to the terror group had launched a counterattack Saturday, hoping to halt ISIS' advance east out of Ramadi toward a key Iraqi military base and the country's capital.
The Iraqis repelled an ISIS attack on the town of Khalidiya and then launched their own offensive to the west
, toward the town of Husayba, which ISIS captured just a day earlier, according to the deputy governor of Anbar province.
Why hasn't Iraq sent in Shiite militias before now?
In his analysis published last week, Lister wrote that the Iraqi government
had been sensitive to allegations of human rights abuses
by Shiite militia in largely Sunni districts during the Tikrit offensive, and aware that the presence of such troops in the Sunni heartland of Anbar might backfire.
ISIS would probably use the arrival of Shiite militia in Anbar as both a warning and rallying call to wavering Sunnis in the province, he said.
As well as fears that Hashd al-Shaabi's involvement could inflame sectarian tensions, its ties to Iran
complicate the use of airstrikes by the U.S. coalition. The militia is backed by Iran.
CNN's Nick Paton Walsh said it was concerning that Hashd al-Shaabi had given the Anbar operation the name "Answering the call of Hussein." Hussein was the son of Ali, the founder of Shiaism, and the name was unlikely to appeal to Sunni tribes, he said.
How are Iraqi Sunnis involved?
Iraqi Deputy Prime Minister Saleh al-Mutlaq, a Sunni who leads his own party and often criticizes Prime Minister Haider al-Abadi, spoke with CNN's Frederik Pleitgen on "Amanpour" on Monday.
"The Sunni people, they are not with ISIS, this is for sure," he said.
But al-Mutlaq suggested that Iraq's Sunnis were uncertain about their future and thus lacking in the determination that had led to the 2007 "Sunni Awakening," when Sunni Iraqis had risen to fight with the U.S. against al Qaeda.
But "they are not certain now if they could fight ISIS, and if they fight ISIS, what is after ISIS? Actually, there is a worry among people -- and among us -- about what is going to happen after ISIS.
"What is after that? Are they going to live in an area which is going to rebuild again? Is there going to be a reconciliation? Are they going to be included in the government? Is there going to be an inclusive government? Is there going to be a real reconciliation in the country? These are our question marks. Without answering them, it will be very difficult to see the end of ISIS in a short time."
How long will the operation last?
In an interview with the BBC, Iraq's Prime Minister said he expected Ramadi to be retaken "in days."
However, CNN's Ian Lee says that Iraq also expected the fight for Tikrit to be short, but the battle lasted for weeks and required the help of U.S. airstrikes.
Walsh said the operation will be complicated.
"This is going to be a complex task -- to get all those groups to read off the same script -- to fight alongside each other, because part of the reason many say ISIS got into Ramadi is the absence of cooperation and coherence of these different groups and their strategy to support that city. And of course ISIS has had well over a week to dig themselves in -- it could be a messy fight.
"We are hearing that those security forces and Hashd al-Shaabi are in a rough semicircle around the south of Ramadi. When they actually move in, that's another question we have to wait to see when and how it is answered."
How are Ramadi's civilian residents coping?
Since the takeover of Ramadi this month, close to 55,000 people have fled
, the United Nations has said. Most of the displaced people headed to Baghdad, about 120 kilometers (75 miles) to the east.
ISIS has brutalized not only its military opponents
but also civilians accused of favoring the government or who don't subscribe to ISIS' brand of Islam
In the Ramadi area, witnesses said ISIS militants summarily executed people in the street whom they accused of working with the government.
"They were killing anyone who they accused of being with the police or the army," one witness told CNN.