Two years after Fallujah became one of the first prizes claimed by ISIS, the Iraqi city is a ghost town where fearful residents turn on one another and resistance is met with unspeakable brutality, according to sources trapped inside the Pittsburgh-sized community just 40 miles west of Baghdad.

The sources, who spoke by phone with FoxNews.com, painted a bleak picture of life under an increasingly brutal and desperate ISIS, as it prepares for an expected assault by Iraqi government forces. Backed by coalition air power and aided by Shia militias, Baghdad has already liberated nearby Ramadi from ISIS, but plans to retake Fallujah fell apart in the latter half of last year when the risk of civilian casualties was deemed too great.

“Nobody can stand up to them. If they speak out, they will be killed."

- Fallujah resident

“They have had almost two years to build up the city for defenses, make strong points, set all sorts of booby traps, dig tunnels for ease of movement between positions,” said former U.S. Army Lt. Col. Daniel Davis, now a foreign policy analyst. “I’d expect they’d fight much more fanatically for Fallujah.”

An uneasy alliance of government forces, Shia militias and Sunni tribal units are now skirmishing with ISIS on the city’s outskirts, signaling the pending -- and likely bloody -- campaign.

Little information has trickled out from Fallujah since it was seized by ISIS in January 2014. The terrorists have informants throughout Fallujah’s population, which once stood at 320,000, but is now unknown, said Davis, who added that ISIS “actively searches for people sending reports of any sort out and will kill them.”

People inside Fallujah told FoxNews.com that ISIS fighters don Iraqi government uniforms to trick citizens into believing the liberation has begun, only to slaughter them when they venture from their homes. Yet skeptics have said previously that much of the brutality is carried out by Shia militias who resist ISIS even as they use it as cover to cleanse the city of the Sunni Muslims they despise.

Such differing accounts are a testament to the mistrust and fear that pervades the city. Under ISIS control, Friday morning prayers are followed by mass executions in the public square, including locking people in cages with ravenous wild animals, blowing them up, setting them on fire and driving armored vehicles over them, say residents.

“Nobody can stand up to them,” an insider currently in the region said. “If they speak out, they will be killed.”

As in other parts of ISIS' sprawling caliphate, women suspected of adultery are beheaded and men believed to be homosexual are thrown from atop buildings.

“All they need is for two witnesses to testify and say that this person has done something wrong and they get killed,” he said, adding that the accusation of adultery against a woman results in beheading. “So two bad people can make something up and have a person killed.”

Two weeks ago, 10 boys “no older than 12” were killed for running away from ISIS training camps, one of the sources inside Fallujah recounted.

Schools are open for a few hours during the day, although boys and girls are strictly separated and the only courses being taught pertain to weapons use, a hard-line interpretation of Islamic doctrine and learning the classical Arabic language which differs from local dialects. ISIS requests that families provide at least one or two child fighters – depending on the size of the family – and boys are forced to register for selection at age 14.

ISIS members infiltrate local communities to root out disloyal residents, generating deep mistrust among members of a community that once lived in harmony, sources said. Scores – and possibly hundreds – of residents have been killed trying to leave Fallujah, and those left behind face constant water, food and electricity shortages. To deter further defection attempts, ISIS strips locals of identity cards and documentations, making prospects for landing in a safer locale far more challenging. 

Those who once served in the Iraqi military or police force are hunted down and killed, as are families that have even a remote connection to the Iraqi government.

A recent and rare case of civilian retaliation came late last month, according to a source in Fallujah, when three ISIS fighters, known by locals as Daesh, were killed by a boy of 13 after they tried to stop him from breeding pigeons.

“They slaughtered 70 birds in front of this boy and flogged the father in front of the family, and when the mother tried to step in the Daesh slapped her – causing major anger for the son,” claimed a witness. The boy is said to have seized one fighter’s AK-47 and gunned down all three of the tormentors.

The boy and his family are now in hiding within their community, the source continued, protected by family and neighbors who despise ISIS, yet hunted by loyalist infiltrators.

Fallujah, like much of Anbar Province, has long been dominated by Sunni Muslims who distrust the current Shia-led government. When ISIS initially took the city, it had substantial popular support from Sunnis, including senior military officers from the reign of Saddam Hussein.

“ISIS does not enter a place and control it firmly unless they have some sort of internal support,” Iraqi-American human rights activist Steven Nabil told FoxNews.com.

But the Sunni terror group’s brutality may be making it even less popular with fellow Sunnis than the hated Shia government.

“Now about 80 percent of the people are against ISIS,” said the source. “And the other 20 percent either support it or a part of it themselves… It used to be more.”