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 payments to the WHO in mid-April, accusing it of being too close to Beijing and covering up and mismanaging the coronavirus pandemic.
On Monday, Trump tweeted images of a letter he sent to WHO Director-General Tedros Adhanom Ghebreyesus, saying the letter was "self-explanatory."
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.
Earlier Monday, the WHO said it would launch an independent review of the response to the coronavirus pandemic.
During a virtual assembly, Tedros acknowledged there had been shortcomings and told the assembly he welcomed calls for a review.
