As per the reports, the Trump administration is working on how to roll out the decision.
He also quipped President Trump "has to look outside his little bubble" when it comes to climate change.
Juncker, who was among the leaders Trump met with last week, suggests the US president thinks he can withdraw from the pact immediately, but notes that it takes several years to pull out of new worldwide treaties.
Junker spoke Wednesday in Berlin.
Axios reported Wednesday that Trump has chose to withdraw from the accord, citing two people with knowledge of the decision.
Asked about Musk's tweet Wednesday, press secretary Sean Spicer told reporters to wait to see what the president does.
The Paris deal is the world's first comprehensive climate agreement, with the aim of keeping the global average rise in temperatures below 2C.
The decision would be a significant foreign policy break with almost every other nation on earth and a major reversal of the Obama administration's efforts on climate change.
Michael Brune, executive director of the environmentalist Sierra Club, denounced the decision. In April, the chief executives of 16 big US corporations, including Apple and Wal-Mart, wrote the White House urging Trump to support the Paris treaty. He also said Trump should "know better" given that he is from New York City and knowing that such a move "would hurt his hometown more than nearly any place else" given that it's an "entirely coastal city".
Obama and a host of other countries signed the climate change agreement in 2015 and the former president touted it as the "best chance we have" to save the planet.
India Prime Minister Narendra Modi (nah-REN'-drah MOH'-dee) and Spain's Mariano Rajoy (mah-ree-AH'-noh rah-HOY') made the comments in a joint statement following talks in Madrid.
Trump had indicated on his worldwide trip last week that he was thinking he would honor his campaign promise to withdraw from the agreement, according to CNN. Another said it's not a done deal.More news: OPEC oil supply cut deal likely to extend
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Meanwhile, influential Republican senator Ted Cruz urged the USA president to withdraw from the Paris Agreement on Climate Change.
"Today, the economy and social aspects are linked to the environmental aspects, but they are also linked to the security aspects, they are linked to the risks of conflict", said Guterres.
He said countries such as China and Canada have and are stepping up and can fill the void left by the U.S.
"Many times I've made the argument that climate action actually creates jobs and creates growth, which is what the United States want, what Canada wants - it's what every country wants", she said.
But other groups found that the impacts could be worse, especially if other countries follow the U.S.'s lead.
"If this is true, it is a big setback".
"When the United States commits to something it abides by the rule of law". A Danish lawmaker from a small, center green party says, "it resembles a crime against humanity and future generations".
The United States is the second biggest carbon-dioxide polluter after China, according to the latest data available from the World Bank. Withdrawing would leave the United States aligned only with Russian Federation among the world's industrialised economies in rejecting action to combat climate change.
"Climate change is undeniable", Guterres said in his speech at New York University Tuesday.
Rajoy and Modi agreed to boost bilateral cooperation in the field of combating climate change.
Sources in the environmental community say European leaders are starting to get calls from the White House laying the groundwork for a US exit.
At a summit meeting of the G7 group of leading economies over the weekend, Mr Trump refused to join the other six leaders in pledging to implement the Paris accord and said he would announce the USA position this week. "AGAIN!" the USA leader said in a tweet late Wednesday, punctuating the post with his populist campaign slogan.
News of Trump's expected decision drew swift reaction from the United Nations.
A White House official said Wednesday that there could be "caveats in the language" announcing a withdrawal, leaving open the possibility that his decision isn't final.