The US exit from the climate pact could raise doubts about the commitment of the world's largest economy to curb global warming and could make it more difficult to hold other nations to their environmental commitments.

All but two countries - Nicaragua and Syria - signed onto the 2015 accord, which was a signature diplomatic achievement for former US president Barack Obama.

The European Union's top climate change official said Trump's decision to leave the Paris accord made it "a sad day for the global community".

The EU climate action commissioner, Miguel Arias Canete, said in a statement that the bloc "deeply regrets the unilateral decision by the Trump administration" but vowed "the world can continue to count on Europe for global leadership".

Italy, France and Germany said they regretted Trump's decision and dismissed his suggestion that the global pact could be revised.

"We deem the momentum generated in Paris in December 2015 irreversible and we firmly believe that the Paris Agreement cannot be renegotiated since it is a vital instrument for our planet, societies and economies," the leaders of the three countries said in a rare joint statement.

Following his announcement, Trump spoke by phone to German Chancellor Angela Merkel, French President Emmanuel Macron, Canadian Prime Minister Justin Trudeau and British Prime Minister Theresa May to explain his decision.

Macron told Trump that the Paris climate deal could not be renegotiated and that, while France would continue to work with Washington, it would no longer discuss climate issues with the United States, a French source said.

"The President spoke with President Trump in a five-minute phone call. The exchange was direct," a source close to Macron said.

"The President said that they could continue to talk, but indicated that nothing was renegotiable with regard to the Paris accord. The United States and France will continue to work together, but not on the subject of climate," the source added.

Macron, who made a televised address in French and English, said Trump had "committed an error for the interests of his country, his people and a mistake for the future of our planet".

"I tell you firmly tonight: we will not renegotiate a less ambitious accord. There is no way," said Macron, who took office less than a month ago.

"Don't be mistaken on climate; there is no plan B because there is no planet B."

Merkel and India's leader, Narendra Modi, pledged their support for the climate accord during meetings in Berlin on Wednesday.

Australia remains committed to the Paris Agreement. Environment Minister Josh Frydenberg spoke to Prime Minister Malcolm Turnbull - who is in Singapore - following the announcement, with the pair confirming they still think Australia's targets are achievable.

"Donald Trump's announcement today is obviously very significant but Australia will carry on because as our prime minister has made very clear, when we sign up to international agreements ...we will follow through," Mr Frydenberg told ABC TV on Friday.

Trudeau said on Twitter that he was "deeply disappointed" with the decision.

"Canada is unwavering in our commitment to fight climate change and support clean economic growth," he said.

Five Nordic countries wrote a last-minute letter to Trump, saying the Paris accord was a commitment "to our children".

"We must reduce global warming," the leaders of Denmark, Finland, Iceland, Norway and Sweden said in a short, joint missive. "The effects are already visible in all parts of our planet. It is of crucial importance that all parties stick to the Paris Agreement."

The Prime Minister of Belgium, Charles Michel, called it "a brutal act".

Argentine Bishop Marcelo Sanchez Sorondo, chancellor of the Pontifical Academy of Sciences and the Pontifical Academy of Social Sciences, told the Italian daily La Repubblica that a withdrawal from the agreement amounted to "a disaster for everyone".

Premier Li Keqiang of China, in Berlin for meetings with Merkel, said on Thursday before Trump's decision that his country remained committed to the fight against climate change and to participating in international efforts for a greener world.

China, the world's biggest emitter of carbon dioxide, stands to gain international credit for standing by the Paris Agreement, but it would not be able to fill the void on its own with the US abandoning the treaty.

"China will continue to uphold its commitments to the Paris climate agreement," Li said, confirming a position his country agreed to alongside the United States in 2014, in what proved to be a watershed moment for the ultimate passage of the landmark accord the following year.

Jane J. Chigiyal, ambassador from the Pacific island nation of Micronesia, said her people were already feeling the acute impact. She called sea rise "an existential issue".

"Our contribution to this problem, this challenge, is very small," she said. "Yet we will continue to do our part."

Obama wasted no time in criticising Trump's decision, saying the Trump administration was rejecting the future by pulling out of the climate pact.

Former vice-president Al Gore, who won a Nobel Peace Prize for his work in raising awareness about global warming, said Trump's decision was "reckless and indefensible".

"It undermines America's standing in the world and threatens to damage humanity's ability to solve the climate crisis in time," Gore, who called Trump last month to try to persuade him to keep the United States in the Paris agreement, said in a statement.

"But make no mistake: if President Trump won't lead, the American people will."

The decision was a "major disappointment for global efforts to reduce greenhouse gas emissions and promote global security", United Nations spokesman Stephane Dujarric said.

UN Secretary-General Antonio Guterres "remains confident that cities, states and businesses within the United States - along with other countries - will continue to demonstrate vision and leadership by working for the low-carbon, resilient economic growth that will create quality jobs and markets for 21st century prosperity", Dujarric said.

The US Conference of Mayors said it strongly opposed Trump's action and vowed that American mayors would continue efforts to reduce greenhouse gas emissions blamed for global warming.

Reuters, AAP, AP