"It is in our national interest to helping vulnerable countries to build resilience to climate change," the administration official said as Obama headed to a G20 summit in Australia.

Obama was en route to Brisbane after visiting Myanmar and China, where on Wednesday he and President Xi Jinping announced ambitious targets on greenhouse gas emissions as part of a pact designed to breathe new life into attempts to replace the international Kyoto treaty.

Obama was expected to outline his pledge in a speech on Saturday in Brisbane on the sidelines of the G20 talks, as backers and donors involved in the UN's "Green Climate Fund" (GCF) prepare to meet in Berlin on November 20.

US president's renewed focus on climate change threatens to upend Australia's stated focus of keeping the G20 summit confined to economic issues.

Australian Prime Minister Tony Abbott is a sceptic about man-made global warming and he was forced to deny today that the revived debate about the issue this week risks leaving him isolated.

And Obama himself must yet get Congress to agree to the contribution, which could be a tough sell after the Republicans regained control of both houses in this month's mid-term elections.

The UN-backed GCF is designed to enlist private-sector money on top of government donations, and so help poorer countries to invest in environmentally friendly technologies and build up their defences against rising sea levels and less predictable weather patterns.

US official said Obama's USD 3 billion pledge would also help make the world safer.

"More resilient communities are less likely to descend into instability or conflict in the aftermath of extreme climate events, needing more costly interventions to restore stability and rebuild," the official said.

At the G20 summit, Prime Minister Shinzo Abe is expected to say that Japan will give up to USD 1.5 billion to the Green Climate Fund, Kyodo News agency reported.

Christiana Figueres, the head of the UN Framework Convention on Climate Change (UNFCCC), has called for an initial capitalisation of USD 10 billion by the end of the year.

France and Germany have already pledged to contribute USD 1 billion each.

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