The UN refugee agency (UNHCR) says there has been an "abject denial" around the world of the humanitarian impact of invading Iraq.
Some two million Iraqis are estimated to have fled the country
The UN faces an enormous task in helping countries such as Jordan and Syria cope with the huge influx of Iraqi refugees, a spokesman said.
He said the international community had to step in to help address their food, health and education needs.
Syria says it is home to 1.2m Iraqi refugees, with up to 800,000 in Jordan.
Damascus has repeatedly called for help to deal with the problem.
UNHCR spokesman Peter Kessler said: "There has been an abject denial of the impact, the humanitarian impact, of the war, the huge displacement within Iraq of up to 1.9 million people who are homeless because of the war, and those people who are homeless and never got back to the homes after Saddam Hussein was overthrown."
Many of the refugees need considerable support, and about a quarter of them are children who need education.
Many need food and healthcare, some need counselling because of the violence they have experienced or witnessed, while others need jobs.
"There's a need for governments to come in and address the health, the education, all the needs," Mr Kessler said.
"Food aid needs as well are becoming vital because the population is becoming further and further impoverished since they cannot work.
"So clearly in every area, there's a need to support what the main host governments are doing and then to gird ourselves for what could be, if the war is prolonged, an increasing movement further westwards."
Displaced inside Iraq
On top of that, almost two million more people are displaced inside Iraq - people who have fled their homes to escape the violence.
Jordan has an interest in stopping Iraq from disintegration, for fear that the already high number of refugees going to Jordan will increase substantially
That number, too, is steadily growing, the UN says, with some provinces feeling overwhelmed and attempting to close their boundaries to refugees from other areas.
Many Sunni Arab and Shia people have been forced to flee from mixed areas to districts where their respective communities are in the majority.
A number of Arab Iraqis have moved to the autonomous Kurdish area in the north, where the security problems are less severe.
Most of the people killed in Iraq's violence are men.
Their deaths leave households headed by women who struggle to survive the loss of the main breadwinner, says the BBC's Jill McGivering.
The public distribution system within Iraq is no longer providing a safety net for these people in the way it used to.
All these factors encourage the flow of people into other countries.