Donald Trump is expected to withdraw the US from the Paris climate accord, according to a White House official.
But the official said there may be "caveats in the language" the president uses to announce the withdrawal, leaving open the possibility that the decision is not final.
Nearly 200 nations, including the US, agreed in 2015 to voluntarily reduce their greenhouse gas emissions in an effort to combat climate change.
During Mr Trump's overseas trip last week, European leaders pressed him to keep the US in the landmark agreement.
He had promised during his presidential campaign to pull the US out of the deal.
Mr Trump, who has called global warming a "hoax", tweeted on Wednesday morning: "I will be announcing my decision on the Paris Accord over the next few days. MAKE AMERICA GREAT AGAIN!"
Withdrawing would leave the US aligned only with Russia among the world's industrialised economies in rejecting action to combat climate change.
Word of Mr Trump's decision comes a day after he met Scott Pruitt, the administrator of the Environmental Protection Agency.
Like his boss, Mr Pruitt has questioned the consensus of climate scientists that the Earth is warming and that man-made climate emissions are to blame.
Since taking office, the pair have moved to delay or roll back federal regulations limiting greenhouse gas emissions while pledging to revive long-struggling US coal mines.
What is not yet clear is whether Mr Trump plans to initiate a formal withdrawal from the Paris accord, which under the terms of the agreement could take three years, or exit the underlying UN climate change treaty on which the accord was based.
The US is the world's second largest emitter of carbon, following only China.
Beijing, however, has reaffirmed its commitment to meeting its targets under the Paris accord, recently cancelling construction of about 100 coal-fired power plants and investing billions in massive wind and solar projects.
The news from the White House led to a swift and strong reaction from the United Nations.
The UN's main Twitter page quotes secretary-general Antonio Guterres as saying: "Climate change is undeniable. Climate change is unstoppable. Climate solutions provide opportunities that are unmatchable."
The post includes a link to an impassioned speech Mr Guterres gave on Tuesday when he called on the world to intensify action to combat climate change.
He said in the speech: "It would be very important for the US not to leave the Paris agreement."
