Dr. Fauci cautioned that the results of the study still needed to be properly peer-reviewed, but he was optimistic that remdesivir would become “the standard of care” for patients with Covid-19.

Some scientists were unsettled by the way in which the findings were reported. The disclosure of trial results in a political setting, before peer review or publication, is very unusual, said Dr. Steven Nissen, a cardiologist at the Cleveland Clinic who has conducted dozens of clinical trials.

“Where are the data?” he asked. Scientists will need to see figures on harms associated with the drug in order to assess its benefits, he added.

“This is too important to be handled in such a sloppy fashion,” Dr. Nissen said.

Dr. Michele Barry, a global health expert at Stanford University, said she had faith in Dr. Fauci’s assessment. Still, she added, “It is unusual to call a drug the ‘standard of care’ until peer review of data and publication, and before studies have shown benefit in mortality.”

The Food and Drug Administration is likely at some point to announce an emergency approval for remdesivir, a senior administration official told The New York Times. Another drug touted by the president, hydrochloroquine, also was granted such an approval, but results in patients have been disappointing.

In one study of veterans with Covid-19, those receiving hydrochloroquine and an antibiotic died at higher rates than those given ordinary supportive care.

Mr. Trump also hopes to put in place a crash program to develop a vaccine, an undertaking being seen with skepticism even inside the administration. The accelerated process, known internally as Operation Warp Speed, would aim to produce hundreds of millions of doses by the end of this year.