Indonesia's forestry ministry said Thursday it would conserve nearly half its share of Borneo island, which is covered with dense rainforest, so as to meet a presidential pledge to reduce gas emissions.

President Susilo Bambang Yudhoyono signed a decree authorising conservation of at least 45 percent of its share of Borneo island, officials said, in a nation that is the world's third-worst emitter of greenhouse gases.

"We hope with the decree, Indonesia will be able to meet its target of reducing gas emissions by 26 percent by 2020," forestry ministry secretary general Hadi Daryanto told AFP.

"At least 45 percent of Indonesian Borneo will serve as the lungs of the world," a release by the president's office said.

Global environment organisations put Indonesia as the world's third-worst emitter of greenhouse gases. They say emissions are mainly due to deforestation caused by the giant palm oil and paper industries, both rife with corruption.

Daryanto said the areas protected will be bigger than the 'Heart of Borneo,' a 2007 agreement signed by Indonesia, Malaysia and Brunei to protect 220,000 square kilometres (85,000 square miles) of equatorial rainforest.

Indonesia's half of Borneo covers about 544,000 square kilometres (210,000 square miles). Its forests are home to some of the world's most diverse wildlife, but are under threat from mostly illegal plantations and logging.

Indonesia shares Borneo island with Malaysia and Brunei.