The United Nations' refugee body has appealed for $60m (£30.8m, 45m euros) in emergency aid for those fleeing violence in Iraq.
One in eight of Iraqis have now left their homes, with up to 50,000 people leaving each month, the UNHCR said.
It said the exodus was the largest long-term movement since the displacement of the Palestinians after the creation of Israel in 1948.
The UNHCR estimates that the number of Iraqis living beyond the country's borders as refugees stands at two million and a further 1.7 million live within the borders as displaced people.
But it warns the number of internally displaced - those forced to leave their homes but not the country - could reach 2.7 million by the end of the year.
Of those who have fled Iraq, the UNHCR estimates that up to one million Iraqis are living in Syria; up to 700,000 in neighbouring Jordan; between 20,000 and 80,000 in Egypt and up to 40,000 in Lebanon.
Many of these refugees live in conditions of acute poverty. In Syria, for example, almost a third of Iraqi refugee children do not go to school.
The agency has also urged neighbouring countries who are hosting refugees to keep their borders open because Iraq's continuing violence will fuel further floods of refugees. "Unremitting violence in Iraq will likely mean continued mass internal and external displacement affecting much of the surrounding region," Mr Guterres said.