The global impact of President Donald Trump's plan to unwind US climate change policies is "unclear" and will only emerge over time, the United Nation's top climate official said yesterday.

In an executive order and a budget proposal, Trump has moved to peel back national policies designed to lower US carbon emissions and meet greenhouse gas reduction pledges under the 196-nation UN Paris climate treaty.

"The precise impact on the secretariat and on global climate action linked with these various announcements remains unclear at this juncture," said Patricia Espinosa, Executive Secretary of the UN body that shepherded the Paris deal into existence.

The approval of US federal budgets, which must be voted by Congress, "involves long and complex negotiations," she said in a statement.

"I, like many people and organisations around the globe, are watching these developments with interest."

Trump's plan to ease emissions limits for coal-fired power plants and scrap more stringent vehicle pollution standards almost guarantees that the United States will fail to meet its commitments under the UN pact, according to experts.

The president has also said he does not intend to honour promises made by the administration of his predecessor, Barack Obama, to give billions of dollars in aid to poor, climate-vulnerable nations.