President Donald Trump announced Sunday that the US Food and Drug Administration (FDA) would be issuing an emergency use authorization (EUA) for blood plasma from patients who have recovered from COVID-19 as a treatment for the disease.

Speaking at the White House a day before the Republican National Convention begins in Charlotte, North Carolina, Trump said the authorization "will dramatically expand access" to the treatment.

Trump called the EUA a "truly historic announcement", adding that the so-called convalescent plasma has been proven to reduce mortality by 35 percent, which he called a "tremendous number".

"We're years ahead of approvals if we went by the speed of past administrations," Trump said, adding, "And that includes vaccines."

"We dream in drug development of something like a 35 percent mortality reduction," said Secretary of Health and Human Services (HHS) Alex Azar, speaking after Trump. "This is a major advance in the treatment of patients."

FDA Commissioner Dr Stephen Hahn told reporters earlier that the agency was "encouraged by the early promising data that we've seen".

Hahn made clear the EUA was not the same as FDA approval and that the treatment still needs to undergo randomized clinical trials.

The agency issues an EUA when it "is reasonable to believe that the product may be effective" and "the known and potential benefits outweigh the known and potential risks".

The treatment, which has already been received by more than 70,000 patients, has shown some benefits, but without rigorous trials, questions remain about its actual safety, effectiveness and appropriate dosage. Trials to date have been hampered by delays and trouble finding volunteers.

Dr Peter Marks, the director of FDA's center for biologics, evaluation and research, said the decision was based on data that suggested that when given early in the course of the disease, plasma "can improve outcomes and decrease mortality".

"We're confident that convalescent plasma is safe to use in this setting," he said.

Trump's press secretary has touted the treatment as a "major therapeutic breakthrough", but given the uncertainties, the EUA could generate controversy both inside the government and the wider scientific community.

The authorization comes less than a week after officials at the National Institutes of Health (NIH) staged a rare intervention to stop the FDA from issuing the EUA. NIH officials told The New York Times that more data from randomized controlled trials was needed before an EUA could be issued.

Trump has been pushing officials for the EUA and seemed to accuse federal health officials of deliberately delaying approval for treatments and vaccines until after Election Day.

"I hear great things about (convalescent plasma treatment) ... that's all I can tell you," said Trump on Wednesday at a White House press briefing. "It could be a political decision because you have a lot of people over there who don't want to rush things because they want to do it after Nov 3, and you've heard that one before."

Asked by a reporter Sunday about the delay in issuing the EUA, Trump said, "There are people in the FDA and actually in your larger department (HHS) that can see things being held up and wouldn't mind so much — it's my opinion, a very strong opinion — and that's for political reasons."

Trump has touted the blood plasma treatment as a "beautiful ingredient". Last month the administration launched an $8 million ad campaign to encourage those who have recovered from COVID-19 to donate plasma.

Since the start of pandemic, the FDA has issued dozens of EUAs, mostly for diagnostic tests but also some treatments. But sometimes, as with the antimalaria drug hydroxychloroquine — which Trump personally publicized as a COVID-19 treatment — further studies find them to be ineffective and possibly harmful, and the EUA was rescinded by the FDA.