Ron Redmond, a spokesman for the UNHCR, told journalists in Geneva on Friday: "We fear hundreds of thousands more Iraqis who have waited to see an improvement in the situation are now teetering on the brink of displacement."
Many of those displaced are being helped by host families, local communities and religious groups, not necessarily major international aid organisations.
"The enormous scale of the needs, the ongoing violence and the difficulties in reaching the displaced make it a problem that is practically beyond the capacity of humanitarian agencies," he said.
At least 1.6 million are what the UN calls internally displaced within Iraq and 1.8 million have fled to neighbouring countries such as Jordan, Egypt, Syria, Lebanon and Iran, according to the UNHCR.
Many were uprooted before the US invasion of Iraq in 2003 that toppled Saddam Hussein.
"The enormous scale of the needs, the ongoing violence and the difficulties in reaching the displaced make it a problem that is practically beyond the capacity of humanitarian agencies"
Redmond said that people were now fleeing their homes in increasing numbers.
Attacks by armed men and street violence after Iraq's first post-war elections in 2005 intensified after the bombing of a major Shia shrine in Samarra in February. This prompted many to seek refuge with family, neighbours and even strangers.
Plea for aid
The International Organisation for Migration (IOM) appealed on Friday for $20 million to allow it to keep distributing food, water and other supplies to those uprooted by the conflict, and to the families and communities who are hosting them.
IOM said, based on an assessment of three Iraqi governorates: "A majority of displaced persons live in inadequate shelter, cannot meet monthly rental expenses, or are facing pressures and tensions due to overcrowded conditions with family and friends."
The UNHCR has received only 60 per cent of the $29 million it requested to help displaced Iraqis and some 50,000 Palestinian, Iranian and Syrian refugees living in Iraq, who, Redmond said, were "in an increasingly dire situation".
The agency is reassessing its financing needs for Iraq and met donor governments this week to discuss an increase in aid. No pledges have yet been announced.