F.D.A.’s officials have advised the White House to scale back the booster plan for the time being.

Dr. Janet Woodcock, acting commissioner of the Food and Drug Administration, and Dr. Rochelle P. Walensky, director of the Centers for Disease Control and Prevention, warned the White House on Thursday that their agencies may be able to determine in the coming weeks whether to recommend boosters only for Pfizer-BioNTech vaccine recipients — and possibly only some of them to begin.

The two health officials made their case during a meeting with White House pandemic coordinator Jeffrey D. Zients. Several persons who heard about the meeting said they didn’t know how Mr. Zients replied. But, for months, he has claimed that the White House will always accept the opinion of government experts, no matter where it goes.

When asked about the meeting, a White House spokesman said on Friday, “We always said we would follow the science, and this is all part of a process that is now underway,” adding that the administration was awaiting the F.D.A.’s “full review and approval” of booster shots, as well as a recommendation from the C.D.C.

“When that permission and recommendation are made,” spokesman Chris Meagher said, “we will be ready to implement the plan that our nation’s top doctors designed to keep us ahead of this virus.”

Mr. Biden stated that, pending FDA approval, the government planned to begin administering boosters to those who had had their second injection of the Pfizer-BioNTech or Moderna vaccine at least eight months ago the week of Sept. 20. This would include a large number of health care employees and nursing home patients, as well as some adults over the age of 65. According to administration officials, users of the single-dose Johnson & Johnson vaccine will most likely be issued a second shot soon.

Mr. Biden framed the strategy as yet another tool in the nation’s arsenal against the highly contagious Delta variant, which has driven up infection rates, flooded hospitals with Covid-19 patients, and resulted in an average of more than 1,500 deaths per day over the past week, according to a New York Times database. “The plan is for every adult to get a booster dose eight months after they get their second vaccination,” says the doctor.

However, the disclosure of a late September target date for beginning the booster campaign sent alarm bells within the FDA, possibly playing a role in the departures of two of the agency’s senior vaccine officials, who announced their departures this week.

Dr. Woodcock and Dr. Walensky both contributed to the plan’s development and publicly supported it. According to some public health professionals, doing so placed pressure on scientists considering the evidence for boosters in their particular agencies to support the administration’s plan.

“Those agencies are now in a box,” said Dr. Steven Joffe, a University of Pennsylvania professor of medical ethics and health policy. “We want doctors, scientists, and the public to have confidence in the recommendations and judgments made, and to be able to refer to the F.D.A. and C.D.C. performing their due diligence.”

According to several people familiar with the discussions, Dr. Woodcock privately argued that it was risky to set a firm date for a booster rollout before regulators had a chance to thoroughly review the data, some of which had yet to be submitted by vaccine manufacturers, and decide whether shots were safe and necessary.