President Joe Biden says efforts to evacuate people from Kabul airport are accelerating, with US troops expanding the perimeter around the site.
Many thousands of Afghans have been queuing at the gates in desperate attempts to flee the country after the Taliban swept to power on 15 August.
Mr Biden said on Sunday that the US had flown nearly 28,000 people out of the airport in the past week.
He faces pressure to extend a deadline for evacuations beyond 31 August.
The president said discussions were taking place with leaders of other countries about pushing back the deadline, but that “our hope is we will not have to”.
“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.
“We have made changes, including extending the access area around the airport – the safe zone,” he said, adding: “We have a long way to go and a lot can still go wrong.”
Earlier, the Pentagon said in a statement that 18 commercial planes would be used to help transfer evacuees to third countries from safe sites outside Afghanistan.
At least 20 people have died as thousands queue outside Kabul airport, a Nato official told Reuters news agency, amid reports that some were being crushed to death.
The scene on Sunday was reportedly calmer than in previous days.
K Armed Forces Minister James Heappey said the Taliban were co-operating and marshalling people into queues at Hamid Karzai International Airport, making the process faster for those hoping to leave.
The UK has evacuated 5,725 people since 13 August, the Ministry of Defence (MoD) said late on Sunday.
Mobilising a civilian air fleet
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 the CRAF will help military aircraft to focus on operations in and out of Kabul, the statement added.
“These civil reserve flights will help facilitate the safe transport [of evacuees] 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.
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.
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.
Mr Johnson is expected to urge Mr Biden to keep American forces in Afghanistan beyond 31 August in order to allow evacuation flights to continue.
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.