The world's largest conservation forum opened in South Korea Thursday with warnings that reckless development was ruining the planet's natural health, pushing thousands of species towards extinction.

"In order to save the earth, all nations must work together, recognizing that they are bound by a common destiny," South Korean President Lee Myung-Bak told the opening ceremony of the World Conservation Congress.

Lee said the state of the natural world had been "severely compromised", with unrestrained development reducing biodiversity and nearly 20,000 species facing extinction.

"Separated from nature, we cannot imagine ways to resolve climate change, poverty or shortages of water, food and energy resources," the president said.

More than 8,000 government officials, NGOs, scientists and business chiefs from 170 countries have gathered in the Korean resort island of Jeju for the 10-day congress focusing on the environment and biodiversity.

The quadrennial conference is held by the International Union for Conservation of Nature (IUCN), whose president Ashok Khosla stressed the need for a holistic socio-economic approach to conservation efforts.

"Conservation policies and action cannot succeed for long unless nations and communities use their resources efficiently, distribute the benefits equitably and empower their citizens actively and inclusively," said Kosla.

The conference is taking place against a drumbeat of scientific warnings that a mass extinction looms, as species struggle to survive in a world of depleted habitat, hunting and climate change.

In a report issued at the Rio+20 world summit in June, the IUCN said that out of 63,837 species it had assessed, 19,817 run the risk of extinction.

At threat are 41 percent of amphibian species, 33 percent of reef-building corals, 25 percent of mammals, 20 percent of plants and 13 percent of birds, the update of the prestigious "Red List" said.

Many are essential for humans, providing food and work and a gene pool for better crops and new medicines, it said.

Experts say that only a fraction of Earth's millions of species, many of them microscopic, has been formally identified.

In recent years, biologists have found new species of frogs and birds in tropical forests -- proof that the planet's full biodiversity is only partly known.

"Of the species that we know about, hundreds of extinctions have occurred among birds and dozens among amphibians, and for invertebrates and insects we really do not know what we may have lost," Tim Blackburn, director of the Institute of Zoology at the Zoological Society of London, told AFP last month.

UN members pledged under the Millennium Development Goals to brake the rate of loss in species by 2010, but fell badly short of the mark.

After this failure, they set a "strategic plan for biodiversity" under which they vowed to prevent the extinction of "most known species."

With 11,000 volunteer scientists and more than 1,000 paid staff, the IUCN runs thousands of field projects around the globe to monitor and help manage natural environments.