The US has enlisted commercial planes to help with the evacuation of people from Afghanistan.
A Pentagon statement said the 18 aircraft would not fly to Kabul, but would instead help move people transiting in third countries.
On Sunday, the US Department of Defense announced the activation of the Civil Reserve Air Fleet (CRAF) to help with the evacuation.
This allows the US to mobilise civilian airlines to help in an emergency. It was last used ahead of, and during, the US invasion of Iraq in 2003 and the Gulf War of 1990-1991.
According to the statement, the level one activation is for 18 planes: four from United Airlines; three each from American Airlines, Atlas Air, Delta Air Lines and Omni Air; and two from Hawaiian Airlines.
“Activating CRAF increases passenger movement beyond organic capability and allows military aircraft to focus on operations in and out of Kabul,” it read.
The news comes as White House national security adviser Jake Sullivan announced there were still several thousand US citizens thought to be in Afghanistan.
Speaking to CNN’s State of the Union programme on Sunday, Mr Sullivan said he could not give a precise number, but noted that work was continuing to evacuate them.
Mr Sullivan also described the threat of attacks by the Islamic State group against the airport as “real” and “acute”.
The Taliban’s swift takeover of Afghanistan has shocked its people and the world pressuring most Afghans to flee because of security concern.
Many thousands of Afghans are crowded outside Kabul airport, desperate to flee the country after the Taliban swept to power last week.
At least 20 people have died, a Nato official told Reuters news agency.But reports suggest the scene is calmer on Sunday than in previous days.
UK armed forces minister James Heappey said the Taliban were now marshalling people into queues at Hamid Karzai International Airport, making the process faster for those hoping to leave.
The UK has airlifted out more than 1,700 people in the past 24 hours, he said, as the situation had improved.
The day before, Sky News’s chief correspondent Stuart Ramsay said that people at the front of the crowd of thousands outside the airport were being “crushed to death”, with British soldiers pulling those in most danger from the throng.
But Taliban official Amir Khan Mutaqi has blamed the US for the “evacuation drama” at the airport.
On Sunday, Mr Mutaqi also addressed potential dissatisfaction within Taliban ranks, arguing that “certain decisions are made in the long-term interest” of the movement and its role in Afghanistan.
He said that the Taliban were in talks with “all factions” to reach an agreement on a future government.
Meanwhile UK Prime Minister Boris Johnson has called an urgent meeting for Tuesday of the leaders of the G7 – the world’s seven largest so-called advanced economies.
“It is vital that the international community works together to ensure safe evacuations, prevent a humanitarian crisis and support the Afghan people to secure the gains of the last 20 years,” he tweeted.
Former UK Prime Minister Tony Blair, meanwhile, has harshly criticised the US withdrawal from Afghanistan.
“You look round the world and the only people really cheering this decision are the people hostile to Western interests,” he said.
The US plans a full withdrawal of its troops from Afghanistan on 31 August.
Several allied nations – including the UK – are calling for this deadline to be extended amid the ongoing evacuation, as US soldiers are currently controlling the airport.