 Almost two million people are homeless within Iraq's borders |
Antonio Guterres said there were almost four million displaced people inside Iraq or in neighbouring countries.
He said the international community had been "overwhelmed" by the problem, and needed to do much more to help.
About 1.8 million have left their homes within Iraq, with two million in Syria, Syria, Egypt, Lebanon, Turkey and Iran.
According to figures collated by the UN refugee agency, UNHCR, some 640,000 out of Iraq's population of 26 million fled their homes in the past year.
Speaking in Amman, Jordan, Mr Guterres, the UN High Commissioner for Refugees, praised Jordan and Syria for accepting large numbers of Iraqis.
"The sacrifices made by these countries are remarkable and the international community needs to assume full responsibility in supporting them," he said.
"They are having huge strains in their infrastructure... we need to help them cope with this massive challenge."
Many refugees live in conditions of acute poverty: in Syria, almost a third of Iraqi refugee children do not go to school.
"When you have almost four million people displaced inside the country or in countries around... we are facing a humanitarian disaster," he said.
"This is the biggest movement of displaced people in the Middle East since the Palestinian crisis in 1948," Mr Guterres said, referring to the movement of Palestinians after the establishment of Israel.
Mr Guterres' comments come after aid agencies voiced their concerns about the situation earlier this year.
The UNHCR has also appealed for $60m (£30.8m, 45m euros) in emergency aid, and plans to hold a donor conference in Geneva in April.
