(CNN) — The Department of Defense is mobilizing commercial airline flights to help with the United States’ evacuation efforts in Afghanistan, which has been chaotic as thousands are desperate to leave the country after the Taliban swiftly took control.

The Pentagon announced Sunday that Defense Secretary Lloyd Austin ordered the commander of US Transportation Command to initiate stage one of its Civil Reserve Air Fleet to assist with flying US citizens, Special Immigrant Visa applicants and other vulnerable individuals out of Afghanistan.

Austin’s directive will activate 18 commercial flights to help with the evacuation efforts: three each from American Airlines, Atlas Air, Delta Air Lines and Omni Air; two from Hawaiian Airlines; and four from United Airlines.

The planes will not fly to Hamid Karzai International airport, but instead will “be used for the onward movement of passengers from temporary safe havens and interim staging bases,” the Pentagon said in a news release.

In a sign of the severity and urgency of the situation, this is the third time the program has been activated in its history, with the first two times being Operations Desert Shield/Storm and Operation Iraqi Freedom, according to the Pentagon.

Activating the program will increase “passenger movement beyond organic capability and allows military aircraft to focus on operations in and out of in Kabul,” the Defense Department said.

The Pentagon said it does not anticipate any “major impact” on commercial flights.

CNN has reached out to the airlines for comment.

Delta is scheduled to have multiple relief flights arriving back in the US beginning Monday morning, the airline said Sunday, adding that it had been in contact with the Defense Department for several days leading up to the military’s ask for commercial airline support.

American said it will be ready to deploy “three widebody aircraft,” starting Monday, while United Airlines said that it is activating four Boeing 777-300s and still trying to glean the extent of what’s expected to be a “small” impact on the rest of its operation.

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

Atlas Air is “proud to provide” the Pentagon with “essential passenger services in the region at this critical time. We are doing as much as possible to provide the much needed capacity to support the evacuation efforts,” a spokesperson for the company told CNN.

More than 20,000 people in and around the Kabul airport have been trying to board flights out of the country, amid one of the largest airlifts in history.

Images and reports have emerged of families scaling the airport’s walls, with video of a baby being hoisted over razor wire to a US Marine. About 20 people are believed to have died from stampedes or gunshots in the last week. Seven Afghan civilians died in crushes near the airport Saturday, a spokesperson for the UK’s Ministry of Defense confirmed to CNN.

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