Desperate Islamic State (Isis) militants in the besieged city of Fallujah are "stealing" blood from weak civilians for their wounded fighters, it has been reported.

The embattled city has been under siege for months and an operation by the Iraqi army that began last month, targeted nearby villages where Isis fighters are located. An estimated force of 20,000 Iraqi government troops sits on the edge of the city, 40 miles (64km) west of Baghdad. Inside the city, the United Nations estimates that 90,000 civilians could still be trapped with many already fleeing over the Euphrates River.

Humanitarian agencies have said that escaping civilians have been shot by Isis snipers on the walls of the city. The extremists are holed up after digging a series of tunnels and laying booby traps.

In October 2015, reports from the self-declared caliphate in Syria and Iraq detailed how Isis had began stealing organs and blood from prisoners to give to jihadists.

In April this year it emerged that the cash-strapped extremists were trying to sell their fighters organs for cash.

Witnesses have said Isis demands from civilians on the street and their homes to give their blood for wounded fighters.

"ISIS now have a large number of wounded fighters and is desperate for blood," an Iraqi witness inside the city told FoxNews.com. "Many of the civilians couldn't get even two meals a day for a long time, so they're very ill and weak."

Isis captured Fallujah in January 2014, the first city to fall to the extremists, before taking over much of Iraq's north and west, declaring a caliphate several months later from Mosul.

The symbolic and strategically-positioned city is falling slowly as the extremists have had months to prepare for an onslaught.

"ISIS is using a lot of snipers and plenty of IEDs," said Omar Nazar, who is head of an elite unit in the Iraqi Emergency Response Division. "They have booby-trapped a lot of homes and they are moving civilians around to use them as human shields."

Iraqi forces have broken through three Isis perimeters and taken control of the city's main hospital and a key bridge across the Euphrates. They have also killed regional Isis commander Abu Amir Ansari in the process.

Fighting ISIS
Shi'ite fighters hold an image of the Islamic State flag after clashes with IS militants in Saqlawiya, north of Fallujah, IraqReuters