There is no evidence that the anti-malaria drug hydroxychloroquine — promoted by US President Donald Trump as a defense against COVID-19 — treats or prevents the novel coronavirus, the World Health Organization (WHO) said Wednesday, as the top US infectious disease expert also said it isn't an effective treatment, and two European countries banned its use.

"We do not advise the use of hydroxychloroquine or chloroquine for the treatment of COVID-19 outside randomized control trials or under appropriate close clinical supervision subject to whatever national regulatory authorities have decided," said Dr Mike Ryan, executive director of the WHO's health emergencies program in Geneva. "There is no empirical evidence at this point that these drugs work in this case either for treatment or for prophylaxis."

Earlier Wednesday, Dr Anthony Fauci, a member of the White House Coronavirus Task Force, said he "wasn't so sure'' hydroxychloroquine should be banned "but clearly the scientific data is really quite evident now about the lack of efficacy for it".

Fauci also warned of the risk of "adverse effects" from the drug for some patients with pre-existing heart conditions and other health problems.

France and Italy banned the drug's use on Wednesday following the WHO's announcement Monday that it was pausing a large trial of hydroxychloroquine due to safety concerns.

A regulator in the United Kingdom said Wednesday that a separate trial was also being put on hold, less than a week after its launch. The study, being led by the University of Oxford and partly funded by the Bill and Melinda Gates Foundation, was expected to involve as many as 40,000 healthcare workers.

The WHO's announcement Monday came after a study published last week in The Lancet, a British medical journal, found that hospitalized COVID-19 patients treated with hydroxychloroquine had a higher risk of death than those who didn't take it.

The day before the WHO said it was suspending its drug trial, Trump said he had "just finished" taking a two-week course of the drug to prevent infection.

"And by the way, I'm still here," he said in an interview that aired on Sinclair Broadcasting on Sunday. "Well, I've heard tremendous reports about it," Trump said of hydroxychloroquine. "Many people think it saved their lives. Doctors come out with reports. You had a study in France, you had a study in Italy that were incredible studies."

There are currently no treatments for COVID-19 approved by the US Food and Drug Administration.

Brazil's health ministry said Monday it wouldn't change its recommendation to treat coronavirus with hydroxychloroquine despite the WHO's action. "We're remaining calm, and there will be no change," said Mayra Pinheiro, a health ministry official, at a news conference.

Brazilian President Jair Bolsonaro also has touted the supposed benefits of hydroxychloroquine and a related drug, chloroquine, against the coronavirus.

Brazil's Health Ministry reported that 1,039 people had died from the disease caused by the new coronavirus in the 24 hours through Tuesday evening, while the Centers for Disease Control and Prevention reported 592 deaths in the US on the same day.