By Robert Evans

GENEVA, Sept 26 (Reuters) - Some 480,000 Iraqis have registered as internal refugees or IDPs since the start of 2007, bringing the total in the country to more than 2.25 million, the IOM relief body said on Wednesday.

The IOM, or International Organisation for Migration, said most of those leaving their homes were fleeing sectarian violence -- with 88 percent saying they had moved after being targeted for their religious identity.

"The situation is becoming a displacement catastrophe," Dana Graber Ladek, a Jordan-based official for the IOM, told a news conference. "It is certainly the worst crisis of its type the whole (Middle East) region has seen since 1948."

In that year, hundreds of thousands of Palestinians fled their homes in Arab-Israeli fighting that followed the establishment of the state of Israel.

Graber Ladek said that while there had been little movement between the U.S.-led invasion in March 2003 and early 2006, nearly 1.06 million had moved to other parts of the country since the bombing of a major Shia shrine in February last year.

SURGE IN FLIGHT

That incident, in the city of Samarra, was followed by months of widespread bombings and killings in which Sunni militias and insurgents targeted Shia communities and Shia fighters and militias attacked Sunnis.

Small religious groups, including Christians, have also been attacked, causing many of their members to join both Shias and Sunnis in fleeing abroad -- mainly to Jordan and Syria where there are up to 2.2 million Iraqi refugees.

In U.N. parlance, refugees are defined as people who flee their homes in one country and take up residence in another, while IDPs -- or internally displaced people -- are those who abandon their homes but stay in the same country.

Graber Ladek said the IDPs who had been registered over the last 17 months had joined some 1.2 million who had been dispersed around Iraq during the repressive rule of former dictator Saddam Hussein.

The U.S. effort from February this year to stop the violence with increased troop numbers -- dubbed "the surge" -- had had some success in halting internal displacement in certain regions, like Baghdad, but in others it had continued.

The IOM official, who makes frequent visits to Iraq where her organisation is a key player in assisting the displaced, said the average IDP registration since January had been fairly constant at an average of 60,000 a month.

According to the IOM, which is based in Geneva and cooperates with U.N. agencies, only a minute portion of all Iraqi IDPs live in tented camps.

Most rent substandard housing in new areas or stay with friends and family. But the pressure on already stretched and understaffed social services was placing new burdens on host communities and causing tensions between them and IDPs.

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