The US has formally initiated the process of withdrawing from the World Health Organisation, the UN announced on Tuesday.
The Trump administration sent a letter to the UN providing notice of its exit from the global health group that is leading worldwide efforts to curb the spread of Covid-19.
The notice was sent to UN Secretary General Antonio Guterres on Monday and will take effect on July 6 next year.
That means it could be rescinded under a new administration after the US presidential election in November.
President Donald Trump pledged in late May to end US participation in the WHO, which he criticised for its response to the coronavirus pandemic and accused of bowing to Chinese influence.
The move was immediately attacked by health officials and critics of the administration.
Mr Guterres “is in the process of verifying with the WHO whether all the conditions for such withdrawal are met,” said his spokesman, Stephane Dujarric.
Under the terms of the withdrawal, the US must meet its financial obligations to the WHO before it can be finalised.
The US, which is the agency’s largest donor and provides it with more than $400 million (Dh1.4 billion) a year, owes about $200m in current and past dues.
Mr Trump's presidential rival Joe Biden said on Tuesday he will immediately reverse the decision if elected.
"Americans are safer when America is engaged in strengthening global health. On my first day as President, I will rejoin the @WHO and restore our leadership on the world stage," the presumptive Democratic candidate wrote on Twitter.
The number of coronavirus cases in the US is nearing three million.
More than 130,000 people in the country have died from the illness, which is about a quarter of the global death toll.
Florida, Texas and California are among the about two dozen states that have experienced an alarming rise in infections in the past two weeks, indicating efforts to contain the virus have failed in much of the country.
Meanwhile, the WHO on Tuesday said there was "emerging evidence" that the coronavirus might spread by air farther than previously thought.
It said the pandemic was still accelerating.
The agency said it would issue a new scientific brief within days, after an international group of scientists concluded the virus could travel far beyond two metres.
The two-metre physical distancing guideline has been a major element in the fight against Covid-19, which has killed more than 540,000 people and infected almost 11.7 million since it was identified in China last December.
Meanwhile, WHO director general Dr Tedros Adhanom Ghebreyesus said the pandemic was showing no signs of slowing down, after 400,000 new cases were reported at the weekend.
It took 12 weeks for the world to reach the first 400,000 Covid-19 cases.
"We've clearly not reached the peak of the pandemic," Dr Tedros said.
"While the number of deaths appears to have levelled off globally, in reality some countries have made significant progress in reducing the number of deaths, while in other countries deaths are still on the rise."
The virus has "taken the world hostage", he said.