President Donald Trump threatened to permanently freeze US funding to the World Health Organization unless "substantive improvements" were made within the next 30 days.
Washington suspended funding to the WHO in mid-April, accusing it of being too close to Beijing and covering up and mismanaging the coronavirus pandemic.
In the letter, Trump lists what he says are examples of the WHO's shortcomings in managing the pandemic, including ignoring early reports of the emergence of the virus, and being too close to China.
"It is clear the repeated missteps by you and your organization in responding to the pandemic have been extremely costly for the world. The only way forward for the World Health Organization is if it can actually demonstrate independence from China," Trump said in the letter.
"If the World Health Organization does not commit to major substantive improvements within the next 30 days, I will make my temporary freeze of United States funding to the World Health Organization permanent and reconsider our membership in the organization," he said.
Trump's threat came after the World Health Organization said it would launch an independent review of its response to the coronavirus pandemic.
US President Donald Trump has branded the global body a "puppet of China".
Washington is locked in an increasingly bitter spat with Beijing over the new coronavirus pandemic and has also taken aim at the WHO, which on Monday kicked off its first ever virtual assembly.
A resolution tabled by the European Union called for an "impartial, independent and comprehensive evaluation" of the international response to the pandemic, which has so far infected nearly 4.8 million people and killed more than 317,000.
WHO Director-General Tedros Adhanom Ghebreyesus acknowledged there had been shortcomings and told the virtual assembly he welcomed calls for a review.