The UN Office for the Coordination of Humanitarian Affairs said the disaster had eclipsed the scale of the devastating 2004 Indian Ocean tsunami, the 2005 earthquake in Pakistan and the January 2010 earthquake in Haiti put together.

About 1.5 million people have been evacuated in the south and 1.5 million hectares of valuable farmland destroyed in central Punjab province, while the worst hit has been the northwest, already struggling with Taliban violence.

"We will soon issue an... appeal for several hundred million dollars to respond to immediate needs," UN chief Ban Ki-moon announced, stressing that medium- and long-term assistance "will be a major and protracted task."

UN special envoy Jean-Maurice Ripert said hundreds of millions of dollars would be needed to address the urgent humanitarian needs and billions for the rehabilitation and reconstruction of infrastructure and livelihoods.

Just hours after the appeal was announced, the United States raised its pledge to the flood-hit region from US$35 to 55 million.

AFP