Donald Trump has called for an anti-malaria drug to be used to treat the coronavirus "immediately", despite his top health expert having said that evidence for its effectiveness was so far "anecdotal".

On Saturday morning the president tweeted: "HYDROXYCHLOROQUINE & AZITHROMYCIN, taken together, have a real chance to be one of the biggest game changers in the history of medicine. The FDA has moved mountains - Thank You! Hopefully they will BOTH (H works better with A, International Journal of Antimicrobial Agents) be put in use IMMEDIATELY. PEOPLE ARE DYING, MOVE FAST, and GOD BLESS EVERYONE! @US_FDA @SteveFDA @CDCgov​ @DHSgov."

Hydroxychloroquine is an anti-malarial drug while azithromycin is an antibiotic.

Mr Trump had mentioned hydroxychloroquine as a possible treatment on Thursday, before being publicly rebuked by Anthony Fauci, his lead expert in the fight against Covid-19. The coronavirus has infected at least 18,000 people in the US and killed more than 270.

On Thursday the president said: "I do want to say – to me, something so big – the FDA has approved compassionate use for a significant number of patients. We have a drug called chloroquine. A derivation would be hydroxychloroquine, which I hear even better about.

"It's a common malaria drug. It's been available, so therefore the safety level we understand very well. It's been relatively safe. And it showed very encouraging early results. Really encouraging.

"If we – if this works as well as – hopefully it might. The FDA, which would have taken normally much longer to do under our great Secretary ... the head of the FDA has been – Dr Stephen Hahn – he has been fantastic. He got it approved very quickly. I won't even tell you how quickly, but let's put it this way: It's approved. And we're encouraging you to take a look at it. We have ordered a lot of it, and you can too. It's by prescription.

"It's a very powerful drug for malaria and also for various forms of very serious arthritis. But we think it has a very serious – a very good impact on what we're talking about with respect to the virus. So you'll take a look at that. Then you can coordinate with us. But I think, to me, that's a game changer."

But asked about whether the drug was a promising solution, Dr Fauci, director of the National Institute of Allergy and Infectious Diseases, told a White House news conference on Friday that the "the answer is no".

He said that "the evidence you're talking about ... is anecdotal evidence".

"It was not done in a controlled clinical trial. So you really can't make any definitive statement about it."

Dr Fauci told CNN "there's no magic drug".

This is not the first time during the coronavirus crisis that the president has clashed with his health experts. Having repeatedly claimed that a vaccine could be developed within months, Dr Fauci and others were forced to stress that it would take at least a year and perhaps longer than 18 months.