The president says the W.H.O. must change or he will permanently end funding.

President Trump told the director-general of the World Health Organization he would permanently end all funding to the organization if it did not “commit to substantive improvements within the next 30 days,” according to a copy of a letter he posted to Twitter late Monday night.

Mr. Trump also wrote that the United States would reconsider its membership in the W.H.O. because it was “so clearly not serving America’s interests.”