An estimated 400,000 Iraqis are trapped in the ISIS-controlled Old City of west Mosul amid food shortages and growing panic under shelling, the United Nations refugee agency UNHCR said on Thursday.

Many civilians fear fleeing because of Islamic State snipers, but some 157,000 have reached a transit centre since the Iraqi government offensive on west Mosul began a month ago, it said.

"The worst is yet to come, if I can put it this way. Because 400,000 people trapped in the Old City in that situation of panic and penury may inevitably lead to the cork popping somewhere, sometime, presenting us with a fresh outflow of large-scale proportions," Bruno Geddo, UNHCR representative in Iraq, speaking from the transit centre outside Mosul, told a Geneva news briefing.