The United Nations on Friday hit back at Donald Trump after he vowed to scrap the Paris climate deal if elected US president, saying the agreement was critical to saving the planet.

"The Paris agreement is one of the key achievements that world leaders have made in the critical fight that is designed to make sure that this planet is habitable for all of us, for generations to come," UN spokesman Farhan Haq said.

The presumptive Republican presidential nominee vowed in a speech on Thursday to "cancel" the hard-won agreement reached in Paris in December and signed by 175 countries at the United Nations last month.

Trump had in the past said he was "not a big fan" of the Paris deal and that he would seek to renegotiate it, but on Thursday he declared that he would simply scrap it altogether.

"We're going to cancel the Paris climate agreement, and stop all payments of United States tax dollars to UN global warming programs," he said.

UN Secretary-General Ban Ki-moon considers the Paris deal as his proudest accomplishment during his 10-year tenure, which ends in December.

Ban has no plans to reach out to Trump to try to educate him on the benefits of the Paris deal, according to his spokesman.

The UN chief has "spoken very forcefully about the need to take action on climate change and forcefully against the idea of climate change denialism," said Haq.

"The science is clear. It's well-established and it needs to be respected by everyone," he added.

The Paris climate deal sets the goal of limiting global warming to "well below" 3.6 degrees Fahrenheit (two Celsius) above pre-industrial levels, by moving to clean energy.

China and the United States, the world's top polluters, have said they will ratify the agreement this year.

The administration of President Barack Obama plans to ratify the Paris accord with an executive agreement, bypassing the Senate and setting up a complex and difficult process for any future president wishing to pull out.