Commercial planes will be used to help with the evacuation of people from Afghanistan, the United States said.
Eighteen aircraft will transfer people to third countries from safe sites outside Afghanistan, the Pentagon said.
According to the statement, the Civil Reserve Air Fleet (CRAF) level one activation includes: four from United Airlines; three each from American Airlines, Atlas Air, Delta Air Lines and Omni Air; and two from Hawaiian Airlines.
Activating the CRAF will help military aircraft to focus on operations in and out of Kabul, the statement added.
Many thousands of Afghans are crowded outside Kabul airport, desperate to flee the country after the Taliban swept to power on 15 August.
President Joe Biden said on Sunday that the US had evacuated nearly 28,000 people in the past week.
“There is no way to evacuate this many people without pain and the heart-breaking images you see,” Mr Biden told reporters at the White House, adding: “We have a long way to go and a lot can still go wrong.”
“These civil reserve flights will help facilitate the safe transport to third countries,” Mr Biden said on Sunday. “None will be landing in Kabul.”
The president said a series of “processing stations” had been established in more than two dozen countries, where evacuees would be screened and cleared.
“We will welcome these Afghans who have helped America to their new homes because that’s who we are,” Mr Biden added.
At least 20 people have died as thousands queue outside Kabul airport, a Nato official told Reuters news agency, with reports that some were being crushed to death.
The scene on Sunday was reportedly calmer than in previous days.