The Islamic State has reportedly moved civilians to the center of Fallujah, attempting to use residents of the Iraqi city as shields to ward off government recapture, according to humanitarian officials and local leaders.

Fallujah, a mere 40 miles from Baghdad in Anbar province, has remained in the hands of IS since early 2014, after it became the first large city to fall to the militants. On Sunday, Iraqi Prime Minister Haider al-Abadi announced an offensive to retake it, saying "the Iraqi flag will soon be flying in the sky of Fallujah." The decision followed a series of deadly suicide bombings carried out by IS in Baghdad, which claimed the lives of more than a hundred people.

On Monday, government forces and Shiite militias attacked the city, backed by airstrikes from the US-led anti-IS coalition. Abadi said on Sunday that Sunni tribal fighters would take part in the fighting, but it was unclear to what extent they were involved. As shells and air strikes rained down on the city, few civilians were able to leave, and many were moved to central areas by IS militants.

"ISIS has been moving families into the center of the city as shields, and to stop the bombardment from Iraqi and coalition forces, but the bombardment has gone ahead," said Nasr Muflahi, Iraq country director at the Norwegian Refugee Council, using an acronym for the extremist group.

Estimates of how many civilians remain in Fallujah range from several tens of thousands to nearly 100,000. On Monday evening, Muflahi said that only about 70 families had been able to leave in the last 48 hours, and that IS was blocking more from exiting. He added that if fighting worsens, as many as 50,000 people could eventually attempt to flee.

Sheikh Emad Khalaf Hussein, commander of a local Sunni tribal force opposed to IS, said that the assault on the city began at dawn on Monday.

"There are more Daesh defending the city than they expected, including foreign fighters, who Daesh had brought in," he said from the outskirts of Fallujah, using an Arabic term for the group. Many civilians, said the Sheikh, were being used as "shields" by IS fighters. Countering the promises of Abadi, he contended that "no Sunnis are involved in the operation."

The site of brutal fighting and bombardment during the US occupation of Iraq, any recapture of Fallujah will be measured in large part by the involvement of Sunnis in the Sunni-majority city. When government forces and Shiite militias retook the Anbar capital of Ramadi in late 2015, the city was left largely in ruins. "They don't want to repeat what happened in Ramadi," said Muflahi. "Ramadi was basically turned into a pile of rubble."

Lise Grande, the UN humanitarian coordinator in Iraq, said she was "deeply concerned" about the fate of civilians in Fallujah, who could fall victim to "airstrikes, artillery, and crossfire."

"We are deeply worried by reports from people inside Fallujah that Daesh is rounding up families and using them as human shields," she added.

Grande said the UN had been briefed by Iraqi security forces on steps they were taking to reduce harm to civilians, thousands of whom are already trying to flee, according to reports the UN deems credible. She said that if the offensive was similar to that which resulted in the April capture of Hit, a town near Baghdad, "neighborhoods will be given the chance to declare neutrality and given protection." However the battle for Fallujah proceeds, it will doubtlessly add masses to the 3.4 million Iraqis that are already internally displaced. "Now we are looking at another 3 million being displaced," she said, referring to operations against IS.

Even before this week's offensive, Fallujah teetered on the verge of a humanitarian catastrophe. Since March, the city has effectively been under siege from government forces. According to Human Rights Watch (HRW), as of early April, food was selling "at exorbitant prices," including $500 for a bag of sugar, and $750 for a sack of flour. Citing officials in Baghdad, HRW reported that government troops and Shia militias "are keeping shipments of food and other goods from reaching the city."

Christoph Wilcke, a researcher with HRW, said he was worried by the possibility for "indiscriminate firing from governments forces and Popular Mobilization Forces into the city," referring to the Shia militia's organizing body. He added that there was some concern that IS could employ chemical weapons, but said that since Fallujah had been besieged, "production capacity is unclear."

Abadi's decision to attack Fallujah came as a surprise to American officials, who favored targeting the city of Mosul first. To attack Fallujah instead appeared to reflect a deference to Shiite militias that clamored for revenge after suicide bombings targeting Shiite civilians killed more than a hundred in the past month in and around Baghdad.