Human activities are wiping out three animal or plant species every hour and the world must do more to slow the worst spate of extinctions since the dinosaurs by 2010, the United Nations said on Tuesday.

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"We are indeed experiencing the greatest wave of extinctions since the disappearance of the dinosaurs," said Ahmed Djoghlaf, head of the U.N. Convention on Biological Diversity. Dinosaurs vanished 65 million years ago, perhaps after a meteorite struck.

"Extinction rates are rising by a factor of up to 1,000 above natural rates. Every hour, three species disappear. Every day, up to 150 species are lost. Every year, between 18,000 and 55,000 species become extinct," he said.

"The cause: human activities."

A "Red List" of endangered species, however, lists only 784 species driven to extinction since 1500 — ranging from the dodo bird of Mauritius to the golden toad of Costa Rica.

Craig Hilton-Taylor, manager of the list compiled by the World Conservation Union grouping 83 governments as well as scientists and environmental organizations, said the hugely varying figures might both be right, in their way.

"The U.N. figures are based on loss of habitats, estimates of how many species lived there and so will have been lost," he told Reuters. "Ours are more empirical — those species we knew were there but cannot find."

U.N. climate experts say global warming, blamed mainly on human use of fossil fuels, will wreck habitats by drying out the Amazon rainforest, for instance, or by melting polar ice.