(Adds reaction from officials in Finland, France, EU and environmental groups; adds detail on market reaction)
WASHINGTON, May 31 (Reuters) - President Donald Trump will follow through on a campaign pledge to pull the United States out of a global pact to fight climate change, a source briefed on the decision told Reuters, a move that promises to deepen a rift with U.S. allies.
Trump, who has previously called global warming a hoax, did not confirm the decision in a post on Twitter, saying only, "I will be announcing my decision on the Paris Accord over the next few days."
Trump refused to endorse the landmark climate change accord at a summit of the G7 group of wealthy nations on Saturday, saying he needed more time to decide, and promised an announcement this week.
A U.S. decision to withdraw from the accord could further alienate American allies in Europe already wary of Trump and call into question U.S. leadership and trustworthiness on one of the world's leading issues. A pullout also would be one more step by the Republican president to erase the legacy of his predecessor, Democrat Barack Obama, who helped broker the accord and praised it during a trip to Europe this month.
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 sea level rise, droughts and more frequent violent storms.
The accord, agreed on by nearly 200 countries 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 percent from 2005 levels by 2025.
Environmental groups derided the Trump administration's reported decision. The Sierra Club said a U.S. withdrawal from the Paris deal would be a "historic mistake." Friends of the Earth said it would make America the world's "foremost climate villain."
Axios news outlet, which first reported the withdrawal, said details of the pullout are being worked out by a team that includes EPA Administrator Scott Pruitt, an ally of fossil fuel industries. The choice is between a formal withdrawal that could take three years or leaving the U.N. treaty that the accord is based on, which would be quicker but more extreme, according to Axios.
International leaders began reacting to the reports of Trump's plans.
A withdrawal by the United States would be disappointing but the European Union stands ready to take global leadership on the issue, European Commission Vice President Maros Sefcovic said in Brussels.
"There is a much stronger expectation from our partners across the world from Africa, Asia and China that Europe should assume leadership in this effort and we are ready to do that," Sefcovic added.
France's ambassador to the United States, Gerard Araud, said on Twitter that the Paris agreement "doesn't infringe on U.S. sovereignty" and noted that major American corporations had supported the deal.
Finland's Prime Minister Juha Sipila said a U.S. withdrawal would be a big setback, adding that "we must find partners to continue, because this work must not stop."
Trump had vowed during his 2016 presidential 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.
Trump has repeatedly expressed doubts about climate change, at times calling it a hoax to weaken U.S. industry. An overwhelming majority of scientists the human use of fossil fuels for energy is driving climate change.
The decision to withdraw from the climate accord was influenced by a letter from 22 Republican U.S. senators, including Majority Leader Mitch McConnell, calling for an exit, Axios reported.
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.
Canada, the European Union, and China have said they will honor their commitments to the pact even if the United States withdraws. A source told Reuters that India had also indicated it would stick by the deal.
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.
Trump aides including Steve Bannon, Stephen Miller, lawyer Don McGahn and Peter Navarro, along with EPA chief Pruitt, argued hard for leaving the accord. They said the deal would require the U.S. government to regulate greenhouse gas emissions, which would hurt business.
Trump's administration has already begun killing Obama-era climate regulations.
The "stay-in" camp, which included Trump's daughter Ivanka, chief economic adviser Gary Cohn, and Energy Secretary Rick Perry, had argued the United States could reduce its voluntary emission-cuts targets while still keeping a voice within the accord.
Oil majors Shell and Exxon Mobil have also supported the pact along with a number of Republican lawmakers. Several big coal companies, including Cloud Peak Energy, had publicly urged Trump to stay in the deal as a way to help protect the industry's mining interests overseas, though others asked Trump to exit the accord to help ease regulatory pressures on domestic miners.
Both solar- and coal-related exchange traded funds were widely underperforming the overall stock market on Wednesday. America's largest coal miners, FirstSolar fell more than 3 percent. Arch Coal and Peabody Energy were both down more than 2 percent.
(Reporting by Valerie Volcovici; Additional reporting by Doina Chiacu; Editing by Chizu Nomiyama and Nick Zieminski)