PARIS — Taking up the cudgel on climate change in the face of American opposition, President Emmanuel Macron of France threatened Wednesday not to sign any joint statement from the Group of 20 summit in Japan this week unless it deals with the issue, which he called a “red line.”

In a rebuke of American officials, Mr. Macron emphasized the importance of reiterating support for the Paris climate agreement of 2015, from which President Trump has vowed to withdraw the United States.

“If we don’t speak about the Paris Agreement, and if, to come to an agreement in a meeting of 20, we are no longer able to defend our climate goals, it will be without France,” Mr. Macron told a group of French citizens in Tokyo. “It’s simple,” Mr. Macron said. “It will be without France,” he repeated.

“We cannot, at home, be under pressure from our youth, and rightly so,” Mr. Macron said, referring to student marches on the environment in France and elsewhere in Europe. He also alluded to the Trump administration’s rejection of the accord without naming Mr. Trump. “That some won’t sign, that’s their business. But we shouldn’t collectively lose our ambitions,” he said.