Showing increasing signs of desperation, Islamic State (ISIS) militants have resorted to shooting civilians trying to flee the embattled ISIS-held Iraqi city of Fallujah and literally stealing blood from civilians to treat the wounded among them, news sources say.

On Friday, ISIS militants opened fire on people trying to leave Fallujah, killing at least 30 people, many of them women and children, an Iraqi military spokesman told NBC News.

Witnesses have also seen ISIS fighters accosting people on the streets and in their homes and forcing them to give blood for their wounded comrades, Fox News reports.

Some of those who were forced to give their blood were left dying in the streets, one witness said.

The civilians who were fired upon by ISIS fighters were trying to flee the ISIS-held city and reach security forces in southern Fallujah and later to the Amiriyat Al-Fallujah refugee camp, the military spokesman said.

A source in Fallujah told NBC News that the ISIS had ordered women and children to remain in the city.

"ISIS militants told men that if they want to leave the town, so they are free to do that, under one condition, not to take their families with them, and if they tried to do so they would kill them," the source said.

Another source said the ISIS now has "a large number of wounded fighters and is desperate for blood."

"Many of the civilians couldn't get even two meals a day for a long time, so they're very ill and weak," the source told Fox News.

Fallujah, some 30 miles from Baghdad, has been held by ISIS for more than two years now. Some 30,000 to 40,000 civilians are believed to be trapped in Fallujah.

The Iraqi military launched an offensive to retake Fallujah last month. On Wednesday the Iraqi military said its counterterrorism forces have retaken the neighbourhood of Shuhada, less than 3 miles from the city centre.

The Iraqi military said its forces are trying to retake the city without destroying it or adding to the humanitarian crisis. Military officials said their effort to free the city neighbourhood-by-neighbourhood and even door-to-door is being slowed by dug-in ISIS snipers and planted explosives.

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