US President Donald Trump has announced that he is withdrawing the United States from the landmark Paris climate agreement.

It's a move that honours a campaign promise but risks rupturing global alliances and disappointing environmentalists and corporate titans.

"The Paris Accord is a BAD deal for Americans, and the President's action today is keeping his campaign promise to put American workers first," read a White House memo to supporters explaining the President's decision. "The accord was negotiated poorly by the Obama Administration and signed out of desperation."

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

As of last month, 195 United Nations members had signed the 2015 accord, which was a signature diplomatic achievement for President Barack Obama. Nicaragua and Syria are the only two member states who have not signed.

The Paris agreement has long divided the Trump administration, with the President taking much of the spring to make up his mind amid an intense campaign by both sides to influence his decision.

Secretary of State Rex Tillerson and Ivanka Trump, the President's daughter and adviser, are among those who urged him to stay in the deal, arguing it would be beneficial to the United States to remain part of negotiations and meetings surrounding the agreement as a matter of leverage and influence.

White House chief strategist Stephen Bannon and Environmental Protection Agency administrator Scott Pruitt pushed for a withdrawal, which probably can't be finalised until near the end of Trump's term. They, along with other hard-line conservatives, have sought to convince Trump that meeting the terms of the agreement would be harmful to the bottom lines of US businesses and would jeopardise manufacturing jobs, especially in the Midwest and other regions where Trump found deep support in last year's election.

More than 190 nations agreed to the accord in December 2015 in Paris, and 147 have since formally ratified or otherwise joined it, including the United States - representing more than 80 per cent of the world's greenhouse gas emissions.

It's also heavily backed by US and global corporations, including oil giants Royal Dutch Shell, ExxonMobil and BP. Large corporations, especially those operating in international markets, have had years to get used to the idea that there are likely to be reductions on carbon emissions, and they have been adapting their businesses accordingly for some time.

Withdrawing the United States from the agreement could take years due to the accord's legal structure and language, but such a move would weaken its goals almost immediately.

The US is the world's second-largest greenhouse gas emitter and would otherwise have accounted for 21 per cent of the total emissions reductions achieved by the accord through 2030.

The Paris agreement is designed to set the world on a path towards keeping the warming of the planet "well below" a 2C rise above preindustrial temperatures, an amount of warming that scientists would consider "dangerous" climate change.

Hans Joachim Schellnhuber, a physicist who founded the Potsdam Institute for Climate Impact Research in Germany, said the US departing from the deal could make the planet a few tenths of a degree Celsius warmer, assuming that the world carries ahead with its plans while the United States keeps on emitting greenhouse gases as usual. But he emphasised that's still a significant increase - especially if the goal is ultimately to hold the planet to an even more ambitious 1.5C, another target cited in the Paris agreement.

"It would mean the remaining distance between the guardrails and where we stand right now would be halved by the US contribution," Schellnhuber said. "And this is significant, because it's a narrow escape anyway."

Schellnhuber thinks that other countries would not follow the US out of the accord, and instead are likely to keep pushing to cut their emissions, meaning they may be able to offset the United States' departure from the agreement.

But the diplomatic repercussions of a US withdrawal could be vast, as demonstrated when European leaders last week pushed Trump to stay in the climate deal at the G-7 meeting in Italy. Trump appeared unswayed, and a communique coming out of the meeting pointedly failed to include the United States among G-7 countries backing the agreement.

Some environmentalists warned that pulling out of the deal will be disastrous.

"If Donald Trump pulls the United States out of the Paris Agreement he will be committing a traitorous act of war against the American people," billionaire environmental activist and mega-political donor Tom Steyer said.

"Trump has abdicated American leadership and sent a clear message to our allies and enemies alike: In the search for courageous and moral solutions to the challenges of the 21st century, don't count on America to lead."