"Twenty percent of the world's coral reefs have been effectively destroyed or show no immediate prospects of recovery," said the report, issued on the first day of a United Nations environmental conference in Buenos Aires lasting until December 17.

The Status of Coral Reefs of the World 2004 also said that another "24 percent of the world's reefs are under imminent risk of collapse through human pressures, and a further 26 percent are under a longer-term threat of collapse."

"The major emerging threat to coral reefs in the past decade has been coral bleaching and mortality associated with global climate change," it said.

Bleaching is a mass death of corals caused by a sudden rise in ocean temperatures.

Even so, it said some reefs had recovered sharply from a 1998 bleaching which seriously damaged 16 percent of all reefs worldwide, especially in the western Pacific and the Indian Ocean.

"About 40 percent of the reefs that were seriously damaged in 1998 are either recovering well or have recovered," it said. Some of the report's highlights were issued in Bangkok in November.

It said the 1998 global warming had been the most serious in 1 000 years but was likely to happen about every 50 years in future, largely because of heat-trapping gases in the atmosphere from burning fossil fuels in cars, factories or power stations.

Corals are formed by a build-up of limestone skeletons left by tiny marine animals called polyps. The graveyards can become giant structures like the Great Barrier Reef off Australia, colourful homes to thousands of species from sharks to seaweed.

The report said nations around the world should do more to cut pollution, restrict fishing and fight to curb emissions of greenhouse gases like carbon dioxide to protect corals.

The World Wildlife Fund (WWF) environmental group, which took part in the report, urged governments meeting in Buenos Aires to set a goal of limiting a rises in temperatures linked to global warming to 2C.

"To save coral reefs, governments must reduce carbon dioxide emissions quickly, but also create marine protected areas," said Simon Cripps, head of the WWF's global marine programme. Temperatures have risen by 0.6C since the late 1800s.

The report said the major success of the past five years had been strict protection of a third of the Great Barrier Reef by Australia. The United States is taking similar steps off Hawaii and Florida.

But 75 percent of coral reefs are in developing countries where human populations are rising rapidly and millions depend on reefs for food.