With the United States in the throes of a historic public health crisis, President Donald Trump is aiming fire at the institution at the heart of the response - the World Health Organisation.

Skeptics see a mercurial US leader eager to find a new scapegoat for the COVID-19 disaster, but even some US supporters of the UN body agree that it made missteps as it took pains not to upset China.

Much of the US criticism has been centred personally on the WHO's director general, Ethiopian microbiologist Tedros Adhanom Ghebreyesus, whose virtual news conferences have become a staple in coverage of the virus that has infected some 1.5 million people worldwide.

On Tuesday, as cooped-up people around the world cheered on medical workers for World Health Day, Trump threatened to withhold money from the WHO, of which the United States is the largest funder.

"The WHO really blew it," Trump wrote on Twitter.

"For some reason, funded largely by the United States, yet very China centric. We will be giving that a good look."

Secretary of State Mike Pompeo said Wednesday that the WHO "hasn't accomplished what it was intended to deliver" but stopped short of calling for Tedros to step down.

Representative Chris Smith, the top Republican on the House Foreign Affairs subcommittee on global public health, said Trump was not talking about "defunding, as much as a pause" in WHO support to press for an investigation.

"We need answers and the people of the world deserve a WHO that is absolutely transparent," said Smith, a supporter of robust US aid to fight diseases.

"People in my district are dying. People all over the United States and Europe and elsewhere are dying because of what we think was very, very poorly handled, which is probably the most diplomatic way of putting it," he told AFP.

Early praise for China

For the United States, a key criticism is how the WHO, quoting Chinese scientists, initially said there was "no clear evidence" of human-to-human transmission of the mysterious virus found late last year in the metropolis of Wuhan.

Tedros in late January traveled to China to meet President Xi Jinping and praised Beijing's "transparency" and cooperation with the WHO.

Smith said the WHO should have been "dogged" at a time that China was muzzling doctors including Li Wenliang, a COVID-19 whistleblower who later died of the disease.

"When they accepted the Chinese government's line that there was no human-to-human transmission, there were people in Wuhan who were paying a terrible price for speaking truth to power," Smith said.

"It's either a competency issue or there's some other agenda, and with the Chinese government there is always another agenda and the truth is always the first casualty," he said.

Trump himself had said in January that the virus was "totally under control" in the United States, later adding it may go away in April due to warmer temperatures. 

The United States has since recorded more than 14,700 deaths, with most of the population under some form of lockdown.

As criticism mounted of his own response, Trump heavily condemned Beijing over what he called the "Chinese virus." 

But his tone stepped up against the WHO after he spoke to Chinese President Xi Jinping and reached a truce with the Asian power, which supplies much of the medical equipment needed in the United States.

Trump - whose tough line on immigration is a signature issue - in particular has taken umbrage to the WHO's initial criticism of his ban on visitors from China.

The WHO said as of late February that such bans were ineffective and warned of "negative social and economic effects," but much of the world has now imposed major restrictions on travel.

Caught in 'Washington blame game'

J. Stephen Morrison, director of the Global Health Policy Center at the Center for Strategic and International Studies, said the WHO deserved some criticism for its "gushing" praise of China and delay in declaring a public health emergency.

But he said it was important to be realistic about the limited leverage of the WHO, especially when faced with a major power such as China.

"What is going on here, I believe, is that this is part of the Washington blame game," Morrison said.

"This is the WHO getting caught in the middle of a worsening US-China confrontation and an increased effort by Washington to deflect attention away from Trump."

Morrison said that Tedros had proven his competence, including the former Ethiopian foreign minister's successful efforts to fight Ebola in Africa.

"It would have had no backing from the US or other powerful states if it had chosen to be confrontational with China at those key moments," Morrison said of the WHO.

"It would have simply had its access terminated."