LE BOURGET, France (AFP) - The United States vowed on Wednesday (Dec 9) to double the money it spends helping poorer countries deal with the immediate effects of climate change.

"We will not leave the most vulnerable nations among us to, quite literally, weather the storm alone," Secretary of State John Kerry told the UN climate summit in Paris.

American officials said the US budget for so-called climate "adaptation" would double from around US$430 million (S$605 million) in 2014 to more than US$800 million by 2020.

The increase would not come at the cost of Washington's existing climate commitments, but Congress will be asked to re-prioritise State Department and Treasury budgets.

The extra funding will be used for grant-based support for countries, a senior official said, including schemes to protect mangrove forests or improve local weather forecasts.

"Obviously, the world's largest economies - including the United States - need to play a major role," Kerry said.

"That's just common sense. That's why the United States has pledged US$3 billion to the Green Climate Fund.

"It is why the US already contributes more than US$2.5 billion in climate finance annually."