"Climate change threatens to intensify water insecurity on an unparalleled scale," the annual UN Human Development Report said.

"Even with drastic reductions in carbon emissions, past emissions mean that the world now has to live with dangerous climate change."

Higher temperatures and less rainfall will cut water to some of the world's most water-stressed areas, while water flows will become less predictable and more subject to extreme events.

East Africa, the Sahel and southern Africa will see huge rainfall reductions with productivity losses in basic foods.

Projections for rain-fed areas in East Africa -- already suffering damaging drought and hunger -- point to potential productivity losses of up to 33 percent in maize and more than 20 percent for sorghum.

Accelerated glacial melt would lead to rising sea levels and loss of river delta systems, which coupled with low rainfall, would threaten major food systems in South Asia and Egypt.

"We estimate that in the next 25 years the number of people living in water-stressed countries will up from around 800 million to 3 billion people," the report's author Kevin Watkins told reporters.

"We argue that we are heading for an entirely predictable humanitarian catastrophe," Watkins said.

The Adaptation Fund attached to the UN Kyoto Protocol, which sets targets for cutting the greenhouse gases blamed for rising temperatures, would mobilise only US$20 million by 2012 on current projections to help farmers cope with changing weather patterns.

The Global Environment Facility -- the principal multi-lateral mechanism for adapting to climate change risks -- had allocated US$50 million for adaptation between 2005 and 2007.

Aid to agriculture had also fallen rapidly in absolute and relative terms over the past decade.

Assistance for farming in developing countries as a whole had fallen in real terms to US$3.2 billion from US$4.9 billion from 10 years ago, with aid to Sub-Saharan Africa less than half what it was in 1990.

"Reversing these trends will be critical to successful adaptation," the UN report said.