"As of today, the Iraqi government says 434,000 men, women and children have fled their homes since fighting escalated in January this year," said Adrian Edwards, spokesman for the UN high commissioner for refugees.
"However, the full scale of the displacement from this under-reported conflict is unknown, as the Iraqi authorities have had to suspend registration over the past month because of insecurity. UNHCR believes the current figure is now close to 480,000," he told reporters in Geneva.
The crisis began in January with fighting between government forces and rebels in eastern Anbar, and has continued in waves as conflict erupted in different parts of the province.
Displacement jumped last month when fighters deliberately breached a dam in Anbar's Abu Ghraib district, flooding the area and forcing some 72,000 people from their homes, Edwards said.
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That has compounded the displacement crisis because people are struggling to get access to clean water, raising health risks.
In addition, there are fears that more civilians will flee the city of Fallujah, where recent shelling hit a hospital and water plant.
Violence was meanwhile hampering efforts to help those in desperate need.
"We urgently need to ramp up our response which is difficult for three reasons. We have deteriorating security in Anbar hindering access to people in need, the displaced are spread out across much of the country, and donor support is lacking," said Edwards.