The head of the United Nations on Tuesday warned the United States against leaving the Paris climate deal, saying other countries would fill a “void” if President Trump decides to step away from the landmark agreement.

“If one country decides to leave a void, I can guarantee someone else will occupy it,” Secretary-General António Guterres said during an event at New York University on Tuesday.

Guterres warned that international adversaries such as China and Russia could step into a global leadership role if Trump backs off the climate commitments made by former President Obama under the Paris agreement. Guterres noted the growth of clean energy in heavily polluting countries like China and said, "the message is simple: The sustainability train has left the station. Get on the train or get left behind."

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Trump has said he will make a decision on the U.S.’s involvement in the Paris deal sometime this week, after discussing the issue with Group of Seven leaders in Italy on Friday. He has said the Paris deal is unfair to the U.S. because other countries have set less lofty greenhouse gas reduction goals.

Trump is reportedly telling associates he is preparing to pull out of the deal, a step that would put the U.S. out of step with nearly every country in the world. Trump met with Environmental Protection Agency Administrator Scott Pruitt, a Paris critic, on Tuesday.

Guterres said he has urged Trump to stay involved in international environmental work. He said that if the U.S. backs out of Paris, the private sector and local governments should still play a major role.

“It’s very clear that governments are not everything," he said. "On the other hand we are doing our best, in dialog with the administration and with Congress, to make the U.S. understand that funding the development aid, funding foreign policy in general, funding organizations like the UN, is also in the interest of the American people."