More than 715 refugees from around the world who expected to start new lives in the United States have had their flights canceled in recent weeks because President Biden has postponed an overhaul of his predecessor’s sharp limits on new refugee admissions.

Agencies that assist refugees poised to enter the country were notified by the State Department this month that all travel would be suspended until the president sets a new target for admissions this year.

Each year, the president must set a cap on the number of refugees that the United States will admit. Former President Donald J. Trump lowered that number to a historic low of 15,000 for the current fiscal year and placed new restrictions that effectively excluded most applicants from Muslim and African countries.

As a result, tens of thousands of people who have already completed the complex process for resettling in the United States have been stranded abroad, often in overcrowded refugee camps where many have been waiting for years.

“These are refugees approved to come here, who have been waiting for years and whose flights have been canceled numerous times,” said Jennifer Sime, senior vice president for resettlement, asylum and integration at the International Rescue Committee.

“They have a travel date and are then left in precarious circumstances when they are prevented from coming,” Ms. Sime said.

Anticipating a new presidential determination to lift the ceiling on admissions and allow the entry of refugees from countries Mr. Trump had excluded, the State Department began booking refugees on flights last month. But it had to start canceling the flights on Feb. 26, and early this week, with still no action from the president, it suspended all travel, according to an email sent to resettlement agencies.

The delay, for which the White House has offered no explanation, underscores the challenges that Mr. Biden confronts as he seeks to deliver on his pledge to reverse the immigration policies of his predecessor. The president faces a deteriorating situation on the southwestern border, where thousands of young Central American migrants are overwhelming holding facilities in hopes of obtaining asylum in the United States.

“We believe that the administration can both resettle refugees and help those seeking asylum at our border,” said Jenny Yang, senior vice president for advocacy and policy at World Relief, a resettlement agency.

Mr. Biden swiftly showed his commitment to restoring the refugee program after taking office. He reversed a Trump-era executive order that required state and local consent for resettlement of refugees. In his first foreign policy speech, on Feb. 4, the president said he would raise the refugee ceiling to 125,000 in the 2021 fiscal year. Secretary of State Antony J. Blinken notified Congress on Feb. 12 that the administration planned to allow up to 62,500 refugees to enter the country in the fiscal year ending Sept. 30.

An “emergency” report from the State Department stated that “grave humanitarian concerns” justified the lifting of Trump administration restrictions that had prevented even approved refugees from traveling to the United States to reunite with their families. But the president has yet to officially sign off on a new number to allow that travel.

A White House spokesman did not offer a date for when the president would sign a new determination. “While no firm numbers have been finalized, the president’s view is clear,” the spokesman said in a statement. “This program will reflect the generosity and core values of the United States while benefiting from the many contributions that refugees make to our country.”

Until a new ceiling is approved, the limits imposed by Mr. Trump remain in place, and they are preventing most approved refugees from traveling, even though the overall ceiling of 15,000 has not been reached.

This is because of a series of subcategories for refugee slots created by the Trump administration. Priority has been given to Iraqis who had worked for the U.S. military and people, primarily Christians, who are facing religious persecution. The classification system disqualified most other Muslim and African refugees, and it will remain in place until Mr. Biden puts a new ceiling and classification system into place.

“Even though we have a new administration, we are still operating under the previous administration’s framework,” Ms. Sime said.

All told, about 115,000 refugees are in the pipeline to be resettled in the United States. About 33,000 of them have been approved but remain abroad. Only 1,400 arrived in the first three months of the current fiscal year, as of Jan. 31.

The largest number of the world’s latest refugees are from Syria, with more than 592,000 people — 41 percent of the total — awaiting resettlement. Yet just 481 Syrian refugees were admitted to the United States in the 2020 fiscal year, or 4 percent of all those allowed to resettle here.

As a region, Africa has the most displaced people needing resettlement. Yet African admissions to the United States fell from 31,624 in the 2016 fiscal year to 4,160 in the 2020 fiscal year. A total of 38 refugees from South Sudan were admitted in the 2020 fiscal year.

Before Mr. Trump substantially reduced refugee admissions, the last time the United States had significantly closed its doors to refugees was during World War II. An average of 90,000 refugees annually have been admitted each year since 1980, when Congress established by law the refugee resettlement program, co-sponsored by Mr. Biden when he was a senator.

The latest flight cancellations have caused deep distress for many refugees in the United States and abroad.

Before they can board a United States-bound flight, refugees must pass background checks and medical exams, which expire in a matter of months, and many fear they will now have to repeat the grueling process. Many already had faced long processing delays as a result of Covid-19, which halted international travel by American officials who interview refugees. The Trump administration also introduced extra layers of security screening that held up cases.

Joseph Madogo, a refugee from the Democratic Republic of Congo who is living in Tennessee, had been expecting his wife, Diane Taussi, to arrive in Memphis on March 9. He had already rented a new apartment and bought a new bed and dining room set when he heard her flight had been canceled.

“I was so excited and prepared to receive her. I was all ready to surprise her,” said Mr. Madogo, who arrived in the United States five years ago. “Then suddenly, two or three days before, they said she would not make it. I was very, very disappointed.”

Mr. Madogo’s brother, Basuze, a caseworker at World Relief, said he was expecting three friends who have been stuck for years in a camp in Malawi. The men were informed recently that they had been booked on a flight. They gave up their spot at the camp and traveled to the city to take coronavirus tests ahead of their departure date, as required by the United States.

“While they were in a hotel, they got the news their flight was canceled and they had to return to the refugee camp,” Basuze Madogo said. “But when they left the camp, they gave away all their small belongings and the house they had built,” he said. “Now they have nothing.”