These refugees aren't in camps. And that's making it more difficult for aid workers to address their growing needs.
The great majority of Iraqis who have come to Syria have settled in and around the capital. Most have disappeared into the cosmopolitan population of this Middle Eastern hub; many are intentionally keeping their profiles low, for fear of being caught, detained, and sent back to Iraq.
The pattern is the same in Jordan, Lebanon and other Iraqi neighbors.
"It's completely different from a camp situation," says Imran Riza, who represents the U.N. High Commissioner for Refugees in Jordan. "In a camp situation, you know what the population is. You know how much water they need. You know what medical supplies are going in, you know what's being used, you can measure, you can plan."
But with the Iraqis, he says, "to know numbers, to access them, to do proper outreach, to do proper assessments, all these things are far, far more complicated."
Only about 54,000 of the estimated 450,000 to 500,000 Iraqis in Jordan have registered with the U.N. refugee agency.
In Syria, only about 220,000 of an estimated 1.2 million have registered.
Riza blames fear. While he says Jordan has been "quite generous" to the Iraqis - the government has opened up the schools to Iraqis and allowed them the same access to health care that is granted to uninsured Jordanians - it considers the Iraqis guests, not refugees. Many now have overstayed their visas; many are working illegally.
In Syria, the United Nations is working with Iraqi women to identify unregistered refugees in need of services.
"There are always people that fall through the net," says Laurens Jolles, the top U.N. refugee official in Syria. "The more we go out, the more we see that there are more people who need help."