Climate changes and pollution have been blamed for some of the disappearing species, according to the study published by Science magazine Friday.

But some are threatened for no apparent reason, according to researchers who carried out the three year Global Amphibian Assessment.

Of the 5,743 known species of toads, frogs, salamanders, newts and worm like Caecilians, 1,856 (32.5 percent) are under threat, according to the work by 500 researchers in 60 countries.

The study was organised by the World Conservation Union, Conservation International and NatureServe.

Among the species, 122 have completely disappeared since 1980 and scientists fear that hundreds more will become extinct in coming decades.

"Since most Amphibians depend on freshwater and feel the effects of pollution before many other forms of life, including humans, their rapid decline tells us that one of the Earth's most critical life-support systems is breaking down," said Simon Stuart of the GAA.

Tropical zones in central and South America have been worst hit, researchers said.

The unexplained extinctions have been most noted in three protected zones: the Yosemite national park in California, the Monteverde reserve in Costa Rica and the Eungella natiional park in Australia.

Scientists say the amphibians could have fallen victim to a fungicidal illness that has appeared as world temperatures rise.

"During the 1980s, we began to observe decline" in salamander populations said Richard Highton, a University of Maryland biology professor.

"Habitat loss, acid rains, global warming, pesticide positions are possible main reasons for the decline," he said.

Researchers called for efforts to protect the habitat of amphibians and to reproduce the threatened species in captivity.

Numbers of small Australian frogs, the Rheobatrachidae, Leptodactylidae frogs in the American tropics, Bufonidae toads and North American salamanders (Ambystomatidae) have been particularly badly hit.

The Australian frogs have now been wiped out.

The over-exploitation of amphibians for culinary purposes -- frogs legs remain a popular meal in many countries -- has also been highlighted by the researchers.

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