The United States on Tuesday congratulated Indonesia, Malaysia and Brunei on an agreement to protect a swathe of mountainous rainforest covering a third of Borneo. The three countries signed a declaration Monday to protect the "Heart of Borneo" region they share, which is home to endangered orangutans, elephants and rhinos.

The United States hailed a pact "to work together to establish a network of protected areas and to ensure the conservation and effective management of forest resources in the Heart of Borneo region," the US embassy said.

Washington has already pledged 100,000 dollars to support the initiative, which aims to combat illegal logging and wildlife trafficking in the ecologically sensitive area, it said.

Under the declaration, the three countries agreed to work together to conserve about 220,000 square kilometres (88,000 square miles) of equatorial rainforest covering about a third of the island, environmental group WWF said.

WWF said the agreement also ended plans to create the world's largest palm oil plantation in Kalimantan along Indonesia's border with Malaysia.

"The scheme — supported by Chinese investments — was expected to cover an area of 1.8 million hectares and would have had long-lasting, damaging consequences to the 'Heart of Borneo'," it said.

Borneo's rainforests have been under threat from unsustainable logging, forest fires and conversion to plantations.

Since 1996, deforestation across Indonesia has increased to an average of two million hectares (five million acres) a year and now only half of Borneo's original forest cover remains.

Borneo's forests are home to 13 primate species — including endangered orangutans — more than 350 bird species, 150 reptiles and amphibians and about 15,000 species of plants.

Scientists continue to make discoveries in the forests — more than 50 species were found last year alone.