U.S. President Donald Trump has decided to follow through on a campaign pledge to pull the United States out of a global pact to fight climate change, a source who was briefed on the decision told Reuters, a move that should rally his support base at home while deepening a rift with allies abroad.

Trump, who has previously called global warming a hoax, refused to endorse the landmark climate change accord at a summit of the G7 group of wealthy nations, including Canada, on Saturday, saying he needed more time to decide. On Wednesday night, he tweeted that he would announce his decision at 3:00 p.m. on Thursday.

I will be announcing my decision on Paris Accord, Thursday at 3:00 P.M. The White House Rose Garden. MAKE AMERICA GREAT AGAIN! —@realDonaldTrump

The decision to withdraw from the accord was first reported Wednesday by the Axios news outlet, with The Associated Press and Fox News soon citing anonymous sources confirming the pullout.

The decision will put the United States in league with Syria and Nicaragua as the world's only non-participants in the Paris climate agreement.

It could have sweeping implications for the deal, which relies heavily on the commitment of big polluter nations to reduce emissions of gases scientists blame for rising sea levels, droughts and more frequent violent storms.

Trump tweeted earlier on Wednesday that he would announce his decision in "the next few days."

Input from 'a lot of people'

"I'm hearing from a lot of people both ways," Trump told reporters, as he welcomed Vietnamese Prime Minister Nguyen Xuan Phuc to the White House. He said he'd be announcing his decision "very soon."

While Trump currently favours an exit, he has been known to change his thinking on major decisions and tends to seek counsel from a range of inside and outside advisers, many with differing agendas.

A second White House official, who also insisted on anonymity, said Trump had not made a final decision on how to proceed.

Climate change is undeniable<br><br>Climate action is unstoppable<br><br>Climate solutions provide opportunities that are unmatchable.<br>-<a href="https://twitter.com/antonioguterres">@antonioguterres</a> <a href="https://t.co/a8O14NCLUc">https://t.co/a8O14NCLUc</a> —@UN

The accord, agreed on by nearly 200 countries and the European Union in Paris in 2015, aims to limit planetary warming in part by slashing carbon dioxide and other emissions from the burning of fossil fuels. Under the pact, the United States committed to reducing its emissions by 26 to 28 per cent from 2005 levels by 2025.

A senior EU official told the Associated Press on Wednesday the EU and China will reaffirm their commitment to the Paris climate deal on Friday in Brussels, regardless of Trump's decision. The official is involved in preparing the meeting between top EU officials and Chinese Premier Li Keqiang but could not speak on the record because their meeting statement was not finalized.

A source told Reuters that India had also indicated it would stick by the deal.

Prime Minister Justin Trudeau has said Canada's commitment to the deal is unwavering, although at the G7 summit he was not prepared to "lecture another country on what they should do."

The Sierra Club's executive director, Michael Brune, called the expected move by Trump a "historic mistake which our grandchildren will look back on with stunned dismay at how a world leader could be so divorced from reality and morality."

The UN's Twitter page changed Wednesday, soon after the reports that the U.S. was poised to leave the deal, quoting Secretary-General Antonio Guterres as saying, "Climate change is undeniable."

Axios said details of the pullout are being worked out by a team that includes EPA administrator Scott Pruitt. The choice is between a formal withdrawal that could take three years or leaving the UN treaty that the accord is based on, which would be quicker but more extreme, according to Axios.

Citing an unnamed White House official, the Associated Press reported that there may be "caveats in the language" that Trump uses to announce the withdrawal, leaving open the possibility that the decision isn't final, according to the official.

Earlier this year, Trump seemed poised to pull out of NAFTA, but in a matter of hours after those reports emerged, the president said the U.S. would call instead for a major renegotiation of the tripartite trade deal first signed in the early 1990s.

He said conversations with Trudeau and Mexican President Enrique Peña Nieto convinced him, at least in the short term, to not withdraw from the agreement.

Canada committed to climate pact

The decision to withdraw from the climate accord was influenced by a letter from 22 Republican senators calling for an exit, Axios reported. The group included Mitch McConnell, majority leader of the Senate, who is from Kentucky.

Former president Barack Obama, who helped broker the accord, praised the accord during a trip to Europe this month.

The United States is the world's second-biggest carbon dioxide emitter behind China.

Supporters of the climate pact are concerned that a U.S. exit could lead other nations to weaken their commitments or also withdraw, softening an accord that scientists have said is critical to avoiding the worst impacts of climate change.

Trump had vowed during his campaign to "cancel" the Paris deal within 100 days of becoming president, as part of an effort to bolster U.S. oil and coal industries.

That promise helped rally supporters sharing his skepticism of global efforts to police U.S. carbon emissions.

After taking office, however, Trump faced pressure to stay in the deal from investors, international powers and business leaders, including some in the coal industry. He also had to navigate a split among his advisers on the issue.

Hundreds of high-profile businesses have spoken out in favor of the deal, including Apple, Google and Walmart. Even fossil fuel companies such as Exxon Mobil, BP and Shell say the United States should abide by the deal.