A UN climate conference in Bali last year agreed to launch pilot projects to grant poor countries credits for slowing deforestation under a new long-term climate pact beyond 2012. With 28,000 sq km of forests disappearing every year, South and Southeast Asia nations were among the most vulnerable to climate change, the report from the UN's Economic and Social Commission for Asia and the Pacific said.

"If these trends continue, land-use emissions are likely to increase until 2050; much damage will already have been done by the time they start to recede," it said. "Reversing deforestation is thus critical."

Experts say reversing the trend would also help reduce the impact of soil erosion and drought, protect against floods and increase bio-diversity, and thus food security.

The report said governments in Asia and the Pacific were likely to face "eco-refugees" from their own countries and elsewhere in the region, seeking shelter from short term and long term environmental catastrophes.

"These refugees are likely to head to cities and towns, so government needs to plan for this influx both in the short term and long term." it said. "A regional food bank is one measure that countries can adopt for mutual assistance."

The report said it was unfair to expect developing countries to sacrifice growth to cut emissions, but it was also imperative to include them in all mitigating efforts.

"The solution is invest in carbon-reducing technologies," it said.

But the cost of returning greenhouse gas emissions to present levels by 2030 would be about $200 billion annually, the U.N says, through measures such as investing in energy efficiency and low-carbon renewable energy.

The report suggested top polluters such as China and India could held other developing countries in their region develop more practical and affordable green technologies.

"The countries of the region, whatever measure they undertake, need to develop policies that reduce greenhouse gas emissions," it said.