My administration has already started discussions with you on how to reform the organisation. But action is needed quickly. We do not have time to waste, he wrote in his three-page letter.
President Trump tweeted the letter late on Monday night.
That is why it is my duty, as President of the United States, to inform you that, if the WHO does not commit to major substantive improvements within the next 30 days, I will make my temporary freeze of United States funding to the WHO permanent and reconsider our membership in the organisation, Trump said.
"I cannot allow American taxpayer dollars to continue to finance an organisation that, in its present state, is so clearly not serving America's interests, he added.
In his letter dated May 18, Trump alleged that the WHO has failed to publicly call on China to allow for an independent investigation into the origins of the virus, despite the recent endorsement for doing so by its own Emergency Committee.
On Monday, Chinese President Xi Jinping dispelled the criticism of secrecy and cover-up, saying "all along, we have acted with openness, transparency and responsibility. We have provided information to WHO and relevant countries in a most timely fashion.
Trump has accused China of covering up, while Australia and the European Union have called for more transparency in China's COVID-19 control efforts including an independent inquiry into the origins of the virus.
More than 300,000 people have died due to the coronavirus pandemic and over 4.8 million people infected around the world. The US is the worst-hit country with over 90,000 deaths and over 1.5 million confirmed COVID-19 cases, according to Johns Hopkins University.