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

That means the U.S. government will have 100 million doses of the two-shot Pfizer vaccine purchased earlier this year — far fewer than it initially planned — 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 problems 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 if there is a Pfizer shortfall for the second quarter, supplies from other companies may be insufficient to fill the gap, depending on which other vaccines have been authorized by then.

Pfizer officials had urged Operation Warp Speed to initially purchase 200 million doses, or enough for the two-shot regimen for 100 million people last summer, 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, or enough for 50 million people, 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.

When federal officials recently approached Pfizer to buy another 100 million doses for the second quarter of next year — after the company released data showing the shots to be remarkably effective — Pfizer said the company would be able to provide only 50 million doses in the second quarter of the year, and another 50 million doses in the third quarter, the individuals said. That's because other countries have ordered the vaccines, they said.

President Donald Trump is expected to sign an executive order on 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 order will be announced as part of a White House "vaccine summit" designed to highlight the administration's accomplishments on vaccines.

"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 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's is lined up for clearance shortly after that. Shipments of the vaccines will begin within 24 hours of the approvals, federal officials have said.

Warp Speed officials hope other companies with promising vaccine candidates — including Johnson & Johnson — will provide what's needed, but some of those companies are still conducting late-stage clinical trials, while others have not started them.

That means there are several weeks to months before the companies are ready to submit an application to the FDA for emergency authorization. One of the companies, AztraZeneca recently reported vaccine data that was encouraging, but experts raised questions about the data 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 different types of vaccines from different manufacturers. He declined to comment on negotiations with any company, including Pfizer. But he said he did not believe there would be any kind of vaccine "cliff," from which the available doses would fall sharply.

Slaoui said that Johnson & Johnson was likely to 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.

"Specifically this means we can have more Moderna vaccine doses, more Pfizer vaccine doses, more Johnson & Johnson vaccine doses and AstraZeneca doses for the foreseeable future," Slaoui said. "We could have all of them. 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 doses of vaccine, and it contained an option to request up to an additional 500 million doses. Those doses will begin being shipped to communities across the country within 24 hours of FDA approval, which could come as early as this week.

"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," Pfizer spokeswoman Amy Rose said.

Additional doses would be "subject to a separate and mutually-acceptable agreement," she said, and the company would not comment on confidential negotiations that might be taking place with the U.S. government.

No agreements with Moderna beyond its initial contract have been announced, but the U.S. has the option to purchase 400 million additional doses, at a cost of $16.50 per dose.