A Democratic senator says the decision “leaves Americans sick and America alone” as Joe Biden pledges to rejoin if elected.
Donald Trump has moved to officially withdraw the US from the World Health Organisation during the coronavirus pandemic.
The US president has formally notified the United Nations that America is leaving the WHO, prompting his presidential election rival Joe Biden to say he will rejoin immediately if elected.
Mr Trump warned in May that the US was ” terminating it’s relationship” with the health body, accusing it of failing to adequately respond to Covid-19 and being under the “total control” of China.
The news came as both Texas and California confirmed more than 10,000 cases in a single day.
The withdrawal notice was sent to UN Secretary-General Antonio Guterres on Monday and will take effect in a year on 6 July 2021.
It means the withdrawal could be rescinded under a new White House administration.
Mr Biden, the presumptive Democratic presidential nominee, said he would reverse the decision on his first day in office if elected.
He said: “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 Republican chairman of the Senate health committee, Lamar Alexander, said he disagreed with the decision to leave the WHO.
He said: “Certainly there needs to be a good, hard look at mistakes the World Health Organisation might have made in connection with coronavirus, but the time to do that is after the crisis has been dealt with, not in the middle of it.”
Meanwhile, Democratic senator Bob Menendez tweeted: “To call Trump’s response to COVID chaotic and incoherent doesn’t do it justice.
“This won’t protect American lives or interests – it leaves Americans sick and America alone.”
The US State Department said America would continue to seek reform of the WHO, but referred to Mr Trump’s remarks last month when asked if the administration might change its mind.
“I’m not reconsidering, unless they get their act together, and I’m not sure they can at this point,” Mr Trump said at the time.
A spokesman for Mr Guterres said he was “in the process of verifying with the World Health Organisation whether all the conditions for such withdrawal are met”.
Under the terms of the withdrawal, the US must meet its financial obligations to the WHO before it can be finalised.
America, which is the agency’s largest donor and provides it with more than $450m per year, currently owes the WHO $200m in current and past payments.
Mr Trump announced at the end of May that America would be leaving the WHO, less than two weeks after warning the organisation that it had 30 days to reform or lose US support.
The US president has claimed Chinese officials “ignored” their reporting obligations to the WHO and pressured the organisation to mislead the public about the outbreak.
More than 2.9 million coronavirus cases have been recorded in the US, while more than 130,000 people in the country have died with the virus.