The United Nations on Wednesday revised significantly upward the number of civilians believed trapped in besieged Falluja, 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, the U.N. Humanitarian Coordinator for Iraq, Lise Grande, warned that civilians could face a "harrowing" situation in the besieged city 50 kilometers (32 miles) west of Baghdad.

The Iraqi army began an offensive to dislodge the insurgents from Falluja 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 Falluja," 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 operations to help more than 7 million people in the war-torn country, saying only 30 percent of the U.N.'s $860 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."

Iraq's income has shrank dramatically since oil prices collapsed two years ago, restricting its ability to bring relief to more than 3 million internally displaced people, mainly from the northern and western regions under IS control.

Finance Minister Hoshiyar Zebari said on Friday he expected that the recovery of Falluja would take time as the militants had dug tunnels and planted explosive devices in roads and houses to impede the military advance.

But Grande warned that "if Falluja is not retaken quickly, there could be a siege-like situation, and this means the people who are trapped there are going to be in a harrowing situation."

Prime Minister Haider al-Abadi has expressed hope that 2016 will see a "final victory" over Islamic State, with the capture of Mosul, their de facto capital in northern Iraq.