If cleared, Covid shots for children under 5 could be available as soon as June 21, a White House official says.

A top Biden administration health official, anticipating that federal regulators will soon authorize coronavirus vaccines for children younger than 5, said Thursday that the first doses could become available as early as June 21, and that states, pharmacies and community health centers can begin ordering them from the Biden administration on Friday.

Dr. Ashish Jha, President Biden’s coronavirus response coordinator, shared the timeline during an appearance in the White House briefing room. He cautioned that the preparations are contingent on Food and Drug Administration authorization of the doses for children six months through four years old, and a recommendation from the Centers for Disease Control and Prevention. No doses will be shipped until the F.D.A. and the C.D.C. sign off.

“I want to be very clear that I am not here to prejudge the outcome of the process,” Dr. Jha said. “But the administration is hard at work planning all sorts of scenarios based on whatever the outcome is.”

The nation’s 18 million children under 5 are the last group of Americans for whom Covid vaccines are not available, and the frustration among many parents is palpable. Now, for the first time, they have a specific date, albeit a tentative one.

Dr. Jha said that while it will take time for the vaccines to become broadly available, the White House expects that “within weeks” of an authorization, “every parent wants their child to get vaccinated will be able to get an appointment.” He also said he expects that some parents will be reluctant.

Earlier this month, the F.D.A. laid out its own timetable for considering applications from Moderna and Pfizer-BioNTech to authorize doses of their Covid-19 vaccines for young children; a decision on authorization is expected days after an outside panel of experts advising the F.D.A. meets on June 14 and 15.

In making the announcement, Dr. Jha tread a delicate path. Last summer Mr. Biden created an uproar when he announced booster shots would be available to many Americans in mid-September pending F.D.A. approval — only to meet resistance from the agency’s leaders, who said they needed more time to review the data.

But Dr. Jha said it was important for the White House to plan, and to be transparent about that planning. He said the administration has been working closely with local health departments, pediatrician and family doctors, as well as children’s hospitals, and has asked states to distribute the initial tranche of doses to “their highest priority sites, including those serving the highest risk children and hardest to reach areas.”