The US Food and Drug Administration (FDA) said Monday it is revoking its approval for the emergency use of two anti-malaria drugs promoted by President Donald Trump to fight COVID-19, and Trump said Monday that he won't be "shamed" into canceling or postponing his campaign appearance in Tulsa, Oklahoma, on Saturday.

The FDA said that hydroxychloroquine and chloroquine were "unlikely to be effective in treating COVID-19" and that after reviewing data, it determined that the drugs, particularly hydroxychloroquine, didn't demonstrate potential benefits that outweighed the risks.

Trump said last month that he was taking hydroxychloroquine on a preventive basis after two people who work in the White House tested positive for the novel coronavirus.

"All I can tell you is, so far, I feel OK," he told reporters at the time. "It seems to have an impact," he said. "Maybe it does, maybe it doesn't. … You're not going to get sick and die.

"It certainly didn't hurt me," Trump replied Monday when asked by the media about the FDA's withdrawing the emergency authorization for the drug's use.

White House physician Dr Sean Conley had released a memo in May that said that after discussing evidence for and against hydroxychloroquine with Trump, they concluded "the potential benefit from treatment outweighed the relative risks".

In March, the FDA issued the emergency use authorization (EUA) for the drugs. The EUA meant that doctors would be allowed to use them on patients hospitalized with COVID-19 even though the medications hadn't been formally approved by the agency for that disease.

A month later, it warned against taking the drugs to treat COVID-19 outside a hospital or formal clinical trial setting due to the risk that the drugs could cause heart arrhythmias.

According to a letter revoking the FDA's authorization, written by Denise M. Hinton, the FDA's chief scientist, the request to do so came from the Biomedical Advanced Research and Development Authority, a unit of the Department of Health and Human Services in charge of supplying treatments in public health emergencies.

In addition, the agency warned healthcare providers about a newly discovered potential drug interaction related to remdesivir, the only other drug the FDA had authorized to treat COVID-19.

After reviewing new information from large clinical trials, the FDA believed the suggested dosing regimens "are unlikely to produce an antiviral effect", Hinton wrote in a letter announcing the decision.

The Tulsa rally is Trump's first in more than three months. Oklahoma is a reliably Republican state that Trump carried by more than 36 percentage points in 2016, more than doubling the vote the total of his Democratic rival, Hillary Clinton.

The rally comes amid signs that the US could be headed for a resurgence of the coronavirus pandemic as health officials nationwide sound the alarm about a spike in cases amid reopenings, street protests and reports of people flouting social distancing guidelines.

Coronavirus infections in Oklahoma have set new daily records in the last week. Tulsa Health Department Director Bruce Dart warned over the weekend that the Trump rally could ignite a wider crisis.

"(COVID-19) is here in Tulsa, it is transmitting very efficiently," Dart told the Tulsa World newspaper Saturday. "I wish we could postpone this to a time when the virus isn't as large a concern as it is today."

In an editorial published Sunday, the newspaper wrote: "This is the wrong time. This is the wrong place for the rally. … There is no treatment for COVID-19 and no vaccine. It will be our health care system that will have to deal with whatever effects follow."

In a Monday tweet, Trump pointed to the large crowds at protests against racism and police brutality, describing them as "rioters" and "looters" who aren't being held to account by the media for failing to follow public health precautions.

In his tweet saying that almost 1 million people had requested tickets to the rally Saturday night at the BOK (Bank of Oklahoma) Center, which has a seating capacity of 19,000, Trump declared "the Far Left Fake News Media, which had no Covid problem with the Rioters & Looters destroying Democrat run cities, is trying to Covid Shame us on our big Rallies. Won't work!"

Some on social media are encouraging others to register for free tickets for the rally and then not attend to keep seats empty. But the tickets are not for reserved seats, and the BOK Center's doors will open four hours before the event's 8 pm starting time. Those in line will get seats until the arena is full.

Attendees will receive temperature checks, face masks and hand sanitizer before entering, Trump's campaign manager Brad Parscale tweeted Monday. To date, Trump has not worn a mask in public, and those at the rally won't be required to wear face coverings. Parscale also said organizers will take steps to account for the summer heat and provide bottled water to attendees.

In an acknowledgment of the risks the attendees are taking, the campaign is requiring a signed waiver that frees it of any legal liability should someone get the coronavirus at the rally.

The event was originally scheduled for Friday. But Trump moved the date after criticism for planning a rally on Juneteenth — a day commemorating the end of slavery in the US.

Local Juneteenth celebrations had already been canceled before the Trump rally was announced.