(Adds White House spokeswoman, comment from Merkel, UN Secretary General, Kremlin, EU official, Marshall Islands president)
WASHINGTON, June 1 (Reuters) - President Donald Trump on Thursday will announce his decision on whether to keep the United States in a landmark global pact to fight climate change, after a source close to the matter said he was preparing to pull out of the Paris accord.
Trump said on Twitter on Wednesday night he would make the announcement at 3 p.m. (1900 GMT) on Thursday in the White House Rose Garden, ending his tweet with "MAKE AMERICA GREAT AGAIN!"
An American withdrawal, promised by Trump during the 2016 U.S. presidential campaign, would deepen a rift with U.S. allies and align the United States with Syria and Nicaragua as the world's only non-participants in the landmark 195-nation accord agreed upon in Paris in 2015.
The White House remained tight-lipped about Trump's decision. "The president has listened to people from all sides and ultimately he's going to make the best decision for ... the American worker," White House spokeswoman Sarah Sanders said.
The source, speaking on condition of anonymity, said on Wednesday Trump was favoring an exit and was working out terms of the planned withdrawal with U.S. Environmental Protection Agency Administrator Scott Pruitt, an oil industry ally and climate change doubter. The source said any withdrawal announced by Trump could have conditions or caveats that were under discussion.
The Vatican, which under Pope Francis' insistence has strongly backed the accord, would see a U.S. exit as disaster and "a huge slap in the face," Bishop Marcelo Sanchez Sorondo, a senior Vatican official, told the Rome newspaper La Repubblica.
At their meeting last month, the pope gave Trump a signed copy of his 2015 encyclical letter calling for protecting the environment from the effects of climate change and backed scientific evidence that it is caused by human activity.
U.S. supporters of the pact said any pullout by Trump would represent an abdication of American leadership on a leading issue of our time and would show that the United States cannot be trusted to follow through on international commitments.
The pact was the first legally binding global deal to fight climate change. Virtually every nation voluntarily committed to steps aimed at curbing global emissions of "greenhouse" gases. These include carbon dioxide generated from burning of fossil fuels that scientists blame for a warming planet, sea level rise, droughts and more frequent violent storms.
Scientists have said a U.S. withdrawal from the deal could speed up the effects of global climate change, leading to heat waves, floods, droughts and more frequent violent storms.
During the campaign, Trump said the accord would cost the U.S. economy trillions of dollars with no tangible benefit. Trump has expressed doubts about climate change, at times calling it a hoax to weaken U.S. industry.
The Republican vowed at the time to "cancel" the Paris deal within 100 days of becoming president on Jan. 20, part of an effort to bolster U.S. oil and coal industries.
Since taking office, he has come under pressure from some advisers, close U.S. allies, corporate CEOs, Democrats and some fellow Republicans to keep the United States in the accord.
The United States, under former President Barack Obama, had committed to reduce its emissions by 26 percent to 28 percent from 2005 levels by 2025. The United States, exceeded only by China in greenhouse gas emissions, accounts for more than 15 percent of the worldwide total.
Last year was the warmest since records began in the 19th century, as global average temperatures continued a rise dating back decades that leading climate scientists attribute to man-made greenhouse gas emissions that trap heat in the atmosphere.
In Berlin, German Chancellor Angela Merkel, who pressed Trump to stay in the pact last week at a meeting of the G7 industrialized nations, on Thursday described the accord as essential and said she was pleased many other governments agreed.
Merkel met with Chinese Premier Li Keqiang, and they pledged to continue fighting climate change.
China, which overtook the United States as the world's biggest emitter of greenhouse gases in 2007, and the European Union will seek on Friday to save the Paris agreement, with Li meeting top EU officials in Brussels.
In a statement backed by all 28 EU states, the EU and China were poised to commit to full implementation of the agreement, officials said.
The European Union is not considering imposing trade sanctions on the United States if Trump withdraws, EU Climate and Energy Commissioner Miguel Arias Canete said in Copenhagen.
United Nations Secretary-General Antonio Guterres tweeted on Thursday, "Climate action is not just the right thing to do, it is the smart thing to do."
In Moscow, Kremlin spokesman Dmitry Peskov said Russia attaches great significance to the accord, and "the effectiveness of this convention is likely to be reduced without its key participants."
Marshall Islands President Hilda Heine, whose Pacific island nation already is grappling with rising sea levels, expressed disappointment with Trump's expected move but applauded other countries for redoubling their commitments.
"My country's survival depends on every country delivering on the promises they made in Paris - our own commitment to it will never waver," Heine said in a statement.
(Reporting by Valerie Volcovici, Timothy Gardner, Jeff Mason, and Roberta Rampton; Additional reporting by Robin Emmott and Robert-Jan Bartunek in Brussels, Michelle Nichols at the UN; Writing by Will Dunham; Editing by Nick Zieminski)