WASHINGTON — Pfizer has told the Trump administration that it cannot provide substantial additional doses of its coronavirus vaccine until late June or July because other countries have rushed to buy up most of its supply, according to individuals familiar with the situation.

That means the U.S. government may not be able to ramp up as rapidly as it had expected from the 100 million doses of the Pfizer vaccine that it purchased earlier this year, raising questions about whether it can keep to its aggressive schedule to vaccinate most Americans by late spring or early summer.

Trump administration officials denied that there would be availability issues in the second quarter, citing other vaccines in the pipeline, but others said problems are possible.

“I’m not concerned about our ability to buy vaccines to offer to all of the American public,” Gen. Paul Ostrowski, who oversees logistics for Operation Warp Speed, the government’s initiative to expedite vaccine development, said in an interview Monday. “It’s clear that Pfizer made plans with other countries. Many have been announced. We understand those pieces.”

But several officials knowledgeable about the contracts said that supplies from other companies may be insufficient to fill the gap.

Last summer, Pfizer officials had urged Operation Warp Speed to purchase 200 million doses, enough of the two-shot regimen for 100 million people, according to people knowledgeable about the issue who spoke on the condition of anonymity because they were not authorized to discuss the situation. But the Warp Speed officials declined, opting instead for 100 million doses, they said.

“Anyone who wanted to sell us . . . without an (FDA) approval, hundreds of millions of doses back in July and August, was just not going to get the government’s money,” said a senior administration official.

It was only last weekend, with an FDA clearance expected any day, that federal officials reached back out to the company asking to buy another 100 million doses. By then, Pfizer said it had committed the supply elsewhere and suggested elevating the conversation to “a high level discussion,” said a person familiar with the talks.

Pfizer said the company might be able to provide 50 million doses at the end of the second quarter, and another 50 million doses in the third quarter, the individuals said.

The government had a different type of contract with Pfizer than with other companies involved with Operation Warp Speed, the administration’s effort to speed development of vaccines. Pfizer was the only company that did not take government money for research and development, which meant U.S. officials have had less insight into its decisions than it does with the other companies, said a senior administration official familiar with the matter who spoke on the condition of anonymity because the official was not cleared to discuss the issue.

The contract between the government and Pfizer stipulated that if Pfizer’s vaccine were successful and received authorization from the FDA, the U.S. government would purchase 100 million doses at a set price.

Pfizer spokeswoman Amy Rose declined to confirm any information about the company’s discussions with the government, and she said that beyond the first 100 million doses the U.S. has already secured, a separate agreement would have to be negotiated.

President Trump is expected to sign an executive order Tuesday that would prioritize vaccinating Americans before providing doses to other countries, according to a senior administration official who spoke on the condition of anonymity because he was not authorized to speak about the plans. Fox News first reported the executive order. It is not clear whether the order is related to the Pfizer supply issue or whether the president can prevent an American company from fulfilling lawful contracts with other countries.

“The executive order reaffirms to the American people that we are going to put America first,” said a senior administration official, who spoke on the condition of anonymity because he was not authorized to discuss the issue publicly.

The order will be announced as part of a White House “vaccine summit” designed to highlight the administration’s accomplishments on vaccines.

The vaccine by Pfizer and German biotech firm BioNTech is expected to receive emergency authorization from the Food and Drug Administration in the next several days, and Moderna is lined up for likely clearance shortly after that. Shipments of the vaccines will begin within 24 hours of the approvals, federal officials have said.

Warp Speed officials hope that other companies with promising vaccine candidates will supplement the supply in the near future, but some are still conducting late-stage clinical trials, while others have not begun them.

That means it could be several weeks to months before they are ready to apply to the FDA for clearance. One of the companies, AztraZeneca, recently reported encouraging vaccine data, but experts raised questions about it and what it meant about the efficacy of the vaccine.

Moncef Slaoui, chief science adviser to Warp Speed, said in an interview Monday that the U.S. government strategy was to spread its risk widely over many types of vaccines from different manufacturers. He declined to comment on negotiations with any company, but said he did not believe there would be any kind of vaccine “cliff,” from which the available doses would fall sharply.

Slaoui said Johnson & Johnson probably would report trial results in early January and be ready to ship doses in February, if its vaccine is authorized. He predicted that AstraZeneca’s trial would report results in late January or early February and potentially begin providing doses that month.

“We could have all of them,” Slaoui said. “And for this reason, we feel confident we could cover the needs without a specific cliff. . . . We have planned things in such a way as we would indeed avoid a cliff.”

The contract that Pfizer signed with the government in July was to deliver 100 million vaccine doses, and it contained an option to buy an additional 500 million doses.

“Pfizer shall inform the Government of appropriate lead times based on purchase of raw materials, capacity reservation and other factors, and Pfizer and the Government shall mutually agree on an appropriate estimated delivery schedule,” the contract states.

“Recognizing the urgency of the need, our manufacturing teams have been working round-the-clock so we can bring the vaccine to the world as quickly, efficiently and equitably as possible,” Rose said.

Additional doses would be “subject to a separate and mutually acceptable agreement” she said.

No agreements with Moderna beyond its initial contract have been announced, but the U.S. has the option to purchase 400 million additional doses.