The United States has pledged $US1 billion as part of a $US3.5 billion scheme as initial financing towards slowing deforestation, a major contributor to climate change.
Australia, France, Japan, Norway and Britain are also part of the forest protection plan announced at United Nations climate talks in Denmark where world leaders are trying to seal the outlines of a pact to avoid dangerous global warming.
The US government says the money will be contingent on an ambitious political agreement on fighting climate change coming out of the Copenhagen talks.
The money is meant to help fund immediate steps from 2010 to 2012 to develop a UN-backed scheme meant to reward developing nations for saving carbon-rich tropical forests.
The scheme, called reducing emissions from deforestation and degradation (REDD), has had wide support from rich and poor countries in the talks in the Danish capital and kick-start funding has been a key demand from developing nations.
"Protecting the world's forests is not a luxury. It's a necessity," US Agriculture Secretary Tom Vilsack says in the statement from Copenhagen.
"This substantial commitment is reflective of our recognition that international public finance must play a role in developing countries' efforts to slow, halt and reverse deforestation," he says.
Deforestation is responsible for nearly a fifth of mankind's greenhouse gas emissions and curbing forest loss is regarded as a key way to brake the pace of global warming.
Forests such as the vast Amazon jungle or the peat forests of Borneo island in Southeast Asia soak up and lock away large amounts of planet-warming carbon dioxide, acting like lungs of the atmosphere.
REDD is aimed at putting a price on the carbon those forests lock away or are released if cut down, providing a financial incentive to keep them standing.
"This is a very positive and encouraging step," Norwegian Prime Minister Jens Stoltenberg has told Reuters.
"This can help the atmosphere for negotiators in Copenhagen," he says.
The talks have stumbled on emissions targets by rich nations, financing for poor nations and arguments over the final shape of any new legal agreement to fight climate change.
"If we manage to stop deforestation, we'll have averted a third of all emissions we need to cut by 2020," Stoltenberg says.
Norway says it has given more cash to projects for slowing deforestation than any other developed nation.
"This is what's needed to break the logjam of the REDD negotiations here in Copenhagen and spark the additional funding needed to address the global challenge of deforestation," says Andrew Deutz, Director of International Climate Policy for The Nature Conservancy.
"This $US1 billion pledge from the United States should be an appetizer and the US should also serve up the main course for further mitigation and adaptation funding."