The Food and Drug Administration has not yet cleared a coronavirus vaccine for children under 5 because the vaccine manufacturers have not finished their applications for authorization to distribute doses, a top official at the agency suggested on Tuesday.

The official — Dr. Peter Marks, who oversees vaccine regulation for the F.D.A. — said the agency will release a schedule this week for outside expert review of vaccines for the nation’s 18 million children younger than 5. That is the only age group still not eligible for coronavirus vaccination.

Despite growing pressure, including from Congress, the F.D.A. might not rule on whether to authorize a pediatric vaccine dose for that group until June, administration officials have said.

Parents “are frustrated, they are confused, and I am, too,” Senator Patty Murray, Democrat of Washington, who chairs the Senate health committee, told Dr. Marks at an F.D.A. oversight hearing.

Dr. Marks said he was barred from commenting about pending requests from pharmaceutical companies. But he said that vaccine manufacturers usually announce publicly when their applications are finished. “You can surmise what the situation is, because we will proceed with all due speed once we have completed applications,” he said.

Both Moderna and the partnership of Pfizer and BioNTech are developing doses of their coronavirus vaccines for very young children, but Moderna appears to be closer to filing its request for emergency authorization. A Moderna spokeswoman said last week that the company expects to file by the end of April, but did not say whether the company’s application would be complete by then. Pfizer is expected to file its application in May. Many such applications involve rolling submissions of data.

In an interview Thursday with C.N.N.+, Dr. Anthony S. Fauci, the chief medical adviser to the White House, suggested that the F.D.A. wanted to review data from Moderna and Pfizer simultaneously, in order to directly compare the merits of the companies’ vaccines and not “confuse people.” Moderna is proposing a two-shot dosing plan for children under 6, while Pfizer is developing a three-shot vaccination using weaker individual doses for children under 5.

Dr. Fauci’s comment prompted questions about whether the F.D.A. was deliberately holding up Moderna’s request to allow Pfizer to catch up. If so, then regulators would be “failing millions of parents and young children,” Dr. Zachary Rubin, an Illinois pediatric allergist, wrote o. Twitter. “If they are waiting for @pfizer to be ready, then something is rancid.”

In a letter to the F.D.A. on Monday, Rep. James E. Clyburn, Democrat of South Carolina and chairman of a House select subcommittee on the pandemic, wrote that waiting for Pfizer’s application could delay the authorization of Moderna’s vaccine by several weeks. He asked F.D.A. officials to brief the subcommittee’s staff on their plans.

Authorization of a vaccine for the very young is complicated, in part because there is no agreed-upon standard for how effective a vaccine should be against symptomatic infection. In clinical trials, conducted in part during the Omicron surge, neither company’s proposed vaccine performed very well.

Pfizer and BioNTech delayed seeking emergency authorization earlier this year after their trial showed that two doses were less than 50 percent effective against symptomatic disease. The company has said it expects the three-dose plan to be more successful.

Moderna’s vaccine also proved disappointing — it was only about 40 percent effective. The company is studying the effectiveness of a booster dose, and one of its top officials has said she expects a booster to be necessary.