The Vatican, which under Pope Francis' insistence has strongly backed the Paris climate change deal, would see a U.S. exit as a slap in the face and a "disaster for everyone," a senior official said on Thursday.
At their meeting last month, the pope gave U.S. President Donald Trump a signed copy of his 2015 encyclical letter that called for protecting the environment from the effects of climate change and backed scientific evidence that it is caused by human activity.
Vatican Secretary of State Cardinal Pietro Parolin urged Trump in a separate meeting not to quit the Paris accord.
"If he really does (pull out), it would be a huge slap in the face for us," said Bishop Marcelo Sanchez Sorondo, head of the Pontifical Academy of Sciences, which has hosted numerous international conferences on climate change.
"It will be a disaster for everyone," he told the Rome newspaper La Repubblica. In a telephone call with Reuters, Sanchez Sorondo confirmed the comments in the newspaper.
Trump was due to announce his decision later on Thursday. A source briefed on the decision has told Reuters in Washington that he will follow through on his campaign pledge to pull the United States out of the global agreement.
The accord, agreed on by nearly 200 countries in 2015, aims to cut emissions blamed for global warming. The United States committed to reducing its own by 26 to 28 percent, compared with 2005 levels, by 2025.
Sanchez Sorondo said he believed the U.S. oil lobby was behind the decision and that the industry had "maneuvered" Trump.
A withdrawal "would not only be a disaster but completely unscientific," he said. "Saying that we need to rely on coal and oil is like saying that the earth is not round. It is an absurdity dictated by the need to make money."
Trump has called the idea of global warming a hoax.
In March, Cardinal Peter Turkson, the pope's point man for the environment, immigration and development, urged Trump to listen to "dissenting voices" and reconsider his position on climate change.
Former U.S. President Barack Obama helped broker the Paris accord and praised it during a trip to Europe this month.
Canada, the European Union and China have said they will honor their commitments to the pact if the United States withdraws.
(Reporting by Philip Pullella; Editing by Tom Heneghan)