Nearly half a million people have fled their homes since the Iraqi army began its military offensive to drive IS out of Mosul last year, the UN said on Monday.

Following the launch of the operation on October 17, the army was able to recapture the eastern part of the city in January. But fighting between Iraqi forces and IS militants on the part of the city that lies west of the Tigris river has driven out hundreds of thousands of people.

"The sheer volume of civilians still fleeing Mosul city is staggering," Lisa Grande, the UN humanitarian coordinator for Iraq, said in a statement.

Grande said the UN's worst-case scenario in October was as many as one million civilians fleeing Mosul. "Already, more than 493,000 people have left, leaving almost everything behind," she said.

Official: UN at its 'operational limits' 

The UN Office for the Coordination of Humanitarian Affairs (UNOCHA) said that some 500,000 people remained in parts of western Mosul still controlled by IS, which first seized the city in mid-2014.

The UN says food, water and medical supplies are running low in the western part of the city, which still beset with intense fighting. "The battle in western Mosul is very different than in the east - it's much tougher. There are more trauma injuries, homes are being destroyed, food stocks are dwindling quickly and families are at serious risk because there isn't enough drinking water," Grande said.

"Mosul has pushed us to our operational limits," she said.

In March, UN chief Antonio Guterres said during a visit to a displacement camp near Mosul that humanitarian aid efforts were seriously underfunded.