The Pentagon said Sunday it had activated a rarely used program that compels commercial airlines to be used in times of a national emergency, activating 18 civilian aircraft to help evacuate Afghans from Europe and the Middle East.

The orders to the commercial airlines, only the third time in 70 years such a step has been taken, came as part of a furious U.S. effort to ramp up the evacuation as thousands of Afghans crowd Taliban-controlled streets around the international airport in Kabul and amid fears of a terrorist attack.

Among other steps, U.S. officials have designated additional military bases in the U.S. to provide housing for arriving Afghan evacuees, and have eased foreign flight rules to allow overseas carriers to ferry Afghans to the U.S.

Officials said the commercial aircraft wouldn’t fly in and out of Kabul but would ferry evacuees from bases in Germany, Qatar and Bahrain to ease overcrowding pressure and transport bottlenecks.

The Pentagon requested aircraft from six airlines for a period of approximately one to two weeks, defense officials said. American Airlines Group Inc., Atlas Air Worldwide Holdings Inc., Delta Air Lines Inc. and the Omni Air unit of Air Transport Services Group Inc. are providing three aircraft each, while two planes will come from Hawaiian Holdings Inc. and an additional four from United Airlines Holdings Inc.

The move is the latest effort by the Biden administration as it scrambles to transport tens of thousands of Afghans and other civilians out of Kabul after the U.S.-backed government collapsed a week ago, leading to a takeover by the insurgent Taliban movement and a rush of civilians seeking to flee the country.

The U.S. evacuated nearly 8,000 people from the country on about 60 flights over the 24-hour period through early Sunday morning, according to U.S. officials. More than 25,000 people have been evacuated since the airlift began on Aug. 14, the officials said, but thousands more are desperate to leave.

People gathered near a gate at the Kabul airport on Sunday.

Photo: Rahmatullah Alizadah/Zuma Press

Jake Sullivan, the White House national security adviser, said the U.S. military and intelligence agencies are working to prevent potential terrorist attacks and to defend the airport. A State Department alert Sunday warning U.S. citizens and others to avoid airport gates was prompted by a fear of an Islamic State attack, officials said.

“The threat is real. It is acute. It is persistent. And it is something we are focused on with every tool in our arsenal,” Mr. Sullivan said on CNN. “We are taking it absolutely deadly seriously.”

The U.S. military has deployed dozens of C-17 cargo aircraft to fly evacuees out of Kabul. Some flights have transported more than 400 people, packed tightly on the floors of planes designed to carry cargo. Those planes, which lack adequate restroom facilities or the ability to provide food, aren’t suited to carry civilians for the long trip over the Atlantic to U.S. bases, where the Afghans will be taken, officials said.

The Civil Reserve Air Fleet, or CRAF, created after the post-World War II Berlin Airlift, is authorized under the Defense Production Act, which the government has used to compel companies to produce goods and provide services on an urgent basis. The Civil Reserve Air Fleet has been used only twice before, the Pentagon said, during U.S. military operations in Kuwait in 1990 to 1991 and the start of the war in Iraq in 2002 to 2003.

By using commercial aircraft, the U.S. hopes to expand the number of locations to which it can transport Afghans after so far being limited to military installations.

The commercial aircraft used by the military will retain their civilian status and will operate under Federal Aviation Administration regulations, the Pentagon said. But they will be under the control of the U.S. Transportation Command, which is responsible for U.S. military flights, it said.

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President Biden committed Friday to helping stranded Americans and Afghan allies flee Afghanistan following the Taliban’s takeover of the capital. Scenes of desperation continue to play out at Kabul’s international airport. Photo: WAKIL KOHSAR/AGENCE FRANCE-PRESSE/Getty Images The Wall Street Journal Interactive Edition

Carriers were notified through an official warning Friday night that the Civil Reserve Air Fleet could be activated, according to an industry official. The notification could have been rescinded if the Pentagon had decided it didn’t need commercial aircraft to meet its mission.

United Airlines officials said four of their Boeing 777-300 aircraft had been activated. The airline said it expected minimal operational impact but will contact any customers whose flights are impacted.

“We embrace the responsibility to quickly respond to international challenges like these and use our expertise to ensure the safe passage of our fellow countrymen and women as well as those who have risked their lives to help keep them safe,” the airline said in a statement.

On Sunday morning, one of the United planes was en route from Frankfurt, Germany, to an air base in Qatar, according to Flightradar24, a flight tracking site.

American Airlines said it would be ready Monday to deploy three wide-body aircraft to military bases and “other secure transit points” on the Arabian Peninsula and in Europe to assist with emergency evacuations.

“The images from Afghanistan are heartbreaking. The airline is proud and grateful of our pilots and flight attendants, who will be operating these trips to be a part of this lifesaving effort,” the airline said in a statement.

American said it would work to minimize the impact to customers as it temporarily removes the three aircraft from its operation.

Officials with Atlas, which works with the military on cargo flights, said it would fly passenger aircraft, though didn’t specify what kind.

Afghan men and children inside a villa complex in Doha, Qatar, where refugees who recently arrived from Afghanistan were beginning to settle.

Photo: karim jaafar/Agence France-Presse/Getty Images

Delta said it is scheduled to operate multiple flights to the U.S. starting Monday morning. The airline will stage aircraft at various military bases and said it plans to use spare planes, so its commercial operations won’t be affected.

Officials from other airlines didn’t immediately respond to a request for comment.

In discussions last week, airlines had offered to operate charter flights to support the air lift on a voluntary basis, industry officials said.

The timing of the commercial airline activation fits within an Aug. 31 withdrawal deadline set by the Biden administration, although President Biden has said the evacuation effort may continue beyond that deadline to remove all Americans and that officials would consider extending the evacuation for Afghans who worked with U.S. forces or are at risk.

Defense Secretary Lloyd Austin, during an interview Sunday on ABC’s “This Week,” wouldn’t say whether the U.S. would stay beyond the Aug. 31 deadline should all Afghans not be evacuated by then.

In a related move, the Transportation Department issued an order Friday granting foreign carriers that wouldn’t otherwise be allowed to land in the U.S. blanket authority to conduct U.S.-government-sponsored evacuation flights to U.S. airports, until Sept. 30. The order, sought by State Department and Pentagon officials, is aimed at helping move evacuees from overseas military bases to sites in the U.S.

Dulles International Airport, outside of Washington, D.C., is expected to become the central processing site for a surge of Afghan evacuees, officials said.

Pentagon officials are preparing at least one more U.S. base, Joint Base McGuire-Dix-Lakehurst in New Jersey, to begin to receive Afghan refugees. But as the crisis in Kabul unfolds and other nations have grown wary of housing large numbers of Afghan evacuees, the U.S. has begun taking a harder look at its own facilities in the U.S. and overseas, officials said.

A tent city is being erected at the New Jersey base and medical supplies, food, water, restrooms, lighting and other equipment are being installed there now, officials said. Evacuees could be there by next week, they said.

Other bases being studied as potential housing sites include Fort Pickett, Va., Camp Atterbury, Ind., Camp Hunter Liggett, Calif., and Fort Chaffee, Ark. Pentagon officials are also looking at American bases in Japan, Korea, Germany, Kosovo, Bahrain and Italy, officials said.

The Pentagon earlier had identified Fort Lee, Va., Fort Bliss, Texas, and Fort McCoy in Wisconsin as bases that were to begin housing refugees.

—Alison Sider contributed to this article.

Write to Gordon Lubold at Gordon.Lubold@wsj.com and Nancy A. Youssef at nancy.youssef@wsj.com