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.”

It is clear the repeated missteps by you and your organization in responding to the pandemic have been extremely costly for the world,” the president wrote in a four-page letter outlining his grievances against the organization and its leader, Dr. Tedros Adhanom Ghebreyesus.

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Mr. Trump had halted the United States’ contributions to the W.H.O. last month, accusing it of promoting disinformation from China about the outbreak. The organization has denied the claims and insisted that it was transparent and open.

Last year, the United States contributed about $553 million of the W.H.O.’s $6 billion budget, with China providing $43 million. Before Mr. Trump posted his letter, President Xi Jinping of China offered to provide $2 billion in the fight against the pandemic and called on other nations to increase their contributions to the W.H.O.

Alex M. Azar II, the secretary of Health and Human Services, also sharply criticized the W.H.O. on Monday, saying its handling of the outbreak in China led to unnecessary deaths.