Countries around the world reacted with alarm Wednesday after President Donald Trump announced a halt to the sizable funding the United States sends to the World Health Organization. Health experts warned the move could jeopardize global efforts to stop the COVID-19 pandemic, as the United States is the organization's largest single donor.

"We regret the decision of the president of the United States to order a halt in the funding to the WHO," director-general Tedros Adhanom Ghebreyesus said Wednesday at a news briefing.

He went on to say that the organization will assess what impact the withdrawal will have on their work, and then "try to fill any gaps with partners."

Trump has repeatedly labelled COVID-19 the "Chinese virus" and criticized the UN health agency for being too lenient on China, where the novel virus first emerged late last year.

Outside experts have questioned China's reported infections and deaths from the virus, calling them way too low and unreliable. An investigation by The Associated Press has found that six days of delays between when Chinese officials knew about the virus and when they warned the public allowed the pandemic to bloom into a public health disaster.

WATCH | WHO chief responds to Trump plan to halt and review funding:

World Health Organization's Tedros Adhanom Ghebreyesus described the U.S. as a 'generous' friend who had helped the WHO improve the lives of the world's poorest people. 1:40

The European Union on Wednesday said Trump has "no reason" to freeze WHO funding at this critical stage and called for measures to promote unity instead of division. EU foreign policy chief Josep Borrell said the 27-nation bloc "deeply" regrets the suspension of funds, adding that the UN health agency is "needed more than ever" to combat the pandemic.

"Only by joining forces can we overcome this crisis that knows no borders," said Borrell.

Australian Prime Minister Scott Morrison said he sympathized with Trump's criticisms of the WHO, especially its "unfathomable" support of reopening China's "wet markets," where both live and freshly slaughtered animals are sold.

"That said, the WHO, as an organization, does a lot of important work including here in our region in the Pacific and we work closely with them," Morrison told an Australian radio station.

"We are not going to throw the baby out with the bathwater here, but they are also not immune from criticism."

New Zealand Prime Minister Jacinda Ardern is among many world leaders who have criticized Trump's decision to stop funding the WHO during the COVID-19 pandemic. (Mark Mitchell/Getty Images)

New Zealand Prime Minister Jacinda Ardern said WHO was essential to tackling the pandemic.

"At a time like this, when we need to be sharing information and we need to have advice we can rely on, the WHO has provided that," she said. "We will continue to support it and continue to make our contributions."

Germany's foreign minister, Heiko Maas, similarly pushed back, writing on Twitter that "placing blame doesn't help," as "the virus knows no borders."

The Netherlands also threw its support behind WHO.

"Now is not the time to hold back funding. Once the pandemic is under control, lessons can be learned. For now, focus on overcoming this crisis," Sigrid Kaag, minister for foreign trade and development cooperation, said on Twitter.

Devi Sridhar, chair of global public health at the University of Edinburgh, called Trump's decision "extremely problematic," noting that the chronically underfunded WHO is leading efforts to help developing countries fight the spread of COVID-19.

"This is the agency that's looking out for other countries and leading efforts to stop the pandemic," Sridhar said. "This is exactly the time when they need more funding, not less."

WATCH | 'We alerted the world on January the 5th': WHO defends its record on COVID-19:

The World Health Organization has defended its handling of the coronavirus outbreak after President Donald Trump orders withdrawal of U.S. funding for the organization. 2:55

"Trump is angry, but his anger is being directed in a way that is going to ultimately hurt U.S. interests," she said.

Sridhar and others said it was still unclear what the precise impact of Trump's funding cuts might be, but that other health priorities funded by the U.S., like polio eradication, malaria and HIV, were likely to suffer.

China, which has won WHO praise for its actions to curb the virus's spread, urged the United States on Wednesday to fulfil its obligations to the organization.

"This decision weakens the WHO's capability and harms international co-operation," Foreign Ministry spokesperson Zhao Lijian said.

Chinese Foreign Ministry spokesman Zhao Lijian said the country is 'seriously concerned' about Trump's decision to cut funding for WHO. (Carlos Garcia Rawlins/Reuters)

Some experts say WHO dallied in declaring a pandemic. It did so only on March 11, long after the surging outbreaks on multiple continents met the agency's own definition for a global crisis.

In Canada, Conservative Party Leader Andrew Scheer said Wednesday that there are "very serious concerns" about China's influence over the WHO, as well as the accuracy of the organization's data. Scheer went on to criticize the Trudeau government for trusting WHO advisories too closely while planning a response to the pandemic.

"It's incumbent upon this government to explain why they have based so many of their decisions on the WHO," Scheer said.

The UN health agency also insisted for months that the virus could be contained and was not as infectious as influenza. But many outbreak experts, including at the U.S. Centers for Disease Control and Prevention, say COVID-19 is spreading many times faster than flu and that its rate of spread was apparent in January.

Critiques from home

In the United States, Trump's decision has drawn criticism from American Medical Association President Dr. Patrice Harris, the U.S. health advocacy group Protect Our Care, and Melinda Gates, co-chair of the Bill and Melinda Gates Foundation, among others.

U.S. House Speaker Nancy Pelosi called the decision senseless, dangerous and illegal, and said that it "will be swiftly challenged."

Former national security advisor John Bolton took to Twitter to defend the decision, calling it the "correct response" in light of the organization's failures.

Withholding US funding from <a href="https://twitter.com/WHO?ref_src=twsrc%5Etfw">@WHO</a> is the correct response to its coronavirus failures and Chinese influence. It should be a warning flare to the entire UN system that the US will not settle for poor performance <a href="https://twitter.com/hashtag/COVID19?src=hash&ref_src=twsrc%5Etfw">#COVID19</a> —@AmbJohnBolton

Some global health academics said Trump's attacks on WHO might actually strengthen the agency's credibility.

"If Trump was making a great success of the pandemic response in the U.S., if there were minimal cases and deaths there, that might be different," said Sophie Harman, a professor of international politics at Queen Mary University of London. "But things are getting worse, and that reinforces the need for WHO."