The United Nations has revised significantly upward the number of civilians believed trapped in besieged Fallujah, an Islamic State stronghold near Baghdad, to possibly 90,000 from a previous estimate of 50,000.
In a telephone interview with Reuters in Baghdad, UN Humanitarian Coordinator for Iraq Lise Grande warned that the civilians could face a "harrowing" situation in the besieged city 50km west of Baghdad.
The Iraqi army began an offensive to dislodge the insurgents from Fallujah on May 23, but the city has been under a de facto siege, with no supplies getting in, for about six months.
"We have underestimated how many civilians are in Fallujahh," Grande said.
"People who are coming out are giving us the strong impression that we could be talking about maybe 80,000 to 90,000 civilians that are inside."
More than 20,000 people have managed to flee the city in extremely difficult conditions, having walked for days and faced IS fire to reach government held areas, she said.
"A number of them unfortunately didn't make it. We know that more than 10 people have drowned when they tried to cross the river," she said, also reporting cases where families lost their children while fleeing.
Grande made an urgent call to fund the relief operations in Iraq, saying only 30 percent of the UN's $US860 million appeal has been met so far.
"We just can't do more if we don't get urgent funding," she said. "It is so hard to understand how the international community can be investing so much in the military operation and shortchange the victims."