WASHINGTON • President Donald Trump has formally started the withdrawal of the United States from the World Health Organisation (WHO), making good on threats to deprive the UN body of its top funding source over its response to the coronavirus.

Public health advocates and Mr Trump's political opponents voiced outrage at the departure from the Geneva-based body, which leads the global fight on maladies from polio and measles to mental health and Covid-19, at a time when cases have again been rising around the world.

After threatening to suspend the US$400 million (S$558 million) in annual US contributions and then announcing a withdrawal, the Trump administration has formally sent a notice to United Nations Secretary-General Antonio Guterres, a State Department spokesman said on Tuesday.

The withdrawal of the key WHO founding member is effective in one year - on July 6 next year.

Mr Joe Biden, Mr Trump's presumptive Democratic opponent in November elections, vowed he would immediately end the pullout if he won the White House.

"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," he wrote on Twitter.

WHO chief Tedros Adhanom Ghebreyesus responded to the news with a one-word tweet - "Together!" - as he linked to a discussion by US health experts on how leaving the WHO could impede efforts to prevent future pandemics.

In line with conditions set when the WHO was set up in 1948, the US can leave within one year but must meet its remaining assessed financial obligations, UN spokesman Stephane Dujarric said.

In late May, Mr Trump said China exerted "total control" over the WHO and accused the UN body led by Dr Tedros of failing to implement reforms.

Blaming China for the coronavirus, Mr Trump, a frequent critic of the UN, said the US would redirect funding "to other worldwide and deserving, urgent, global public health needs".

Democratic lawmakers have accused Mr Trump of seeking to deflect criticism from his handling of the pandemic in the US, which has had by far the most coronavirus cases and the highest death toll of any nation despite the President's stated hope that the virus would disappear.

"To call Trump's response to Covid chaotic and incoherent doesn't do it justice," said Senator Robert Menendez, the top Democrat on the Foreign Relations Committee.

"This won't protect American lives or interests - it leaves Americans sick and America alone."

Representative Ami Bera, himself a physician, said: "If the WHO is to blame, why has the US been left behind while many countries from South Korea to New Zealand to Vietnam to Germany return to normal?"

Even some of Mr Trump's Republican allies had voiced hope that he was exerting pressure rather than making a final decision to abandon the UN body.

The investigative news outlet ProPublica reported last month that most of Mr Trump's aides were blindsided by the WHO withdrawal announcement, which he made during an appearance about China.

The Trump administration has said that the WHO ignored early signs of human-to-human transmission in China, including warnings from Taiwan - which, due to Beijing's pressure, is not part of the UN body.

While many public health advocates share some criticism of the WHO, they question what other options it had other than to work with China, where Covid-19 was first detected late last year in Wuhan city.

Meanwhile, the US coronavirus task force response coordinator, Dr Deborah Birx, said countries could have had a stronger initial response to Covid-19 if China had been more forthcoming about key features of the virus.

At a panel held by the Atlantic Council, a US think-tank, she said the US would have been more focused on identifying Covid-19 patients without symptoms if China had shared information about the frequency with which patients, particularly young people, are asymptomatic.

"I have to say if we had known about the level of asymptomatic spread, we would have all looked at this differently," she said.

"That's usually the initial countries' responsibility... and I think that did delay across the board our ability to really see or look for this."

Dr Birx said that public health officials had originally assumed that only 15 per cent to 20 per cent of patients are asymptomatic when in fact that number is at least 40 per cent.

"We were looking for people with symptoms. We should have looked for anyone who would have been exposed," she said.

AGENCE FRANCE-PRESSE , REUTERS