In his latest address from the White House, the US president said he still hoped to complete the evacuation from Afghanistan by the August 31 deadline and said he had ordered civilian airlines to assist.
US President Joe Biden said he was still planning to finalise the "heartbreaking" evacuation from Afghanistan by August 31, but left the door open to extending the deadline if necessary.
In a televised address from the White House on the chaotic exit, Biden said his "hope is we will not have to extend" despite a volatile situation on the ground and enormous logistical difficulties as the number of evacuees continues to rise.
"We'll see what we can do," he added when asked by reporters what his reply was to foreign leaders asking for more time. He also warned of potential attacks against the crowded Kabul airport.
Biden said that since last weekend, when the Taliban completed their victorious sweep across Afghanistan by capturing Kabul from the fleeing national government, about 28,000 people have been flown out.
He acknowledged the searing scenes of Afghans crowding the airport in desperation to escape the militants but said this had to be accepted given the situation.
"There is no way to evacuate this many people without pain and loss and heartbreaking images you see," he said.
Underlining why US officials are keen to complete the mission as soon as possible, Biden warned that Daesh extremists pose a constant threat.
"We know that terrorists may seek to exploit the situation," he said. "It's still a dangerous operation."
READ MORE: Afghanistan's Massoud refuses to surrender Panjshir region to Taliban
Civilian airlines ordered to assist in evacuation
In an effort to ramp up the airlift, the US government ordered six airlines - American Airlines, Atlas, Delta, Omni, Hawaiian and United - to provide 18 passenger planes, supplementing an armada of Air Force cargo planes.
The rarely invoked Civil Reserve Air Fleet (CRAF) order will increase the flow of people once they get out of Afghanistan to US bases in Qatar, Bahrain and the United Arab Emirates and are then flown on to third countries in a rapidly expanding global operation. The civilian airliners will not be required to fly to Kabul itself.
Thousands of US troops have been flown in to secure the Afghan capital's airport, while the Taliban control the surrounding city.
The insurgents took over the country so rapidly that the US-led international forces previously propping up the now defunct Afghan national government were caught by surprise - and apparently unprepared to evacuate foreign nationals and Afghan allies.
Biden has faced a torrent of criticism. However, in his remarks from the White House he again defended what he said had been a hard but necessary decision to order the immediate evacuation of Americans, even at the short-term cost of chaos.
"At the end of the day, if we didn't leave Afghanistan now, when would we?" he asked.
Access to airport questioned
Up to 15,000 Americans need to be removed from Afghanistan, according to Biden, who says the administration wants to get at least 50,000 Afghan allies and their family members out of the country.
There have been reports of the Taliban intimidating and beating people trying to reach the airport but US Secretary of Defense Lloyd Austin said the militants had largely been letting American passport holders through safely.
Britain said on Sunday that seven Afghans had died in the chaos near the airport, and the situation was further complicated on Saturday when the US government warned its citizens to stay away from the area because of "security threats."
A White House official said on Sunday the US armed forces had taken out a total of 30,000 people since the end of July with the help of a coalition of US military partners, foreign military and commercial airlines.
In the 24 hours to 0700 GMT (3:00AM), 23 military flights evacuated around 3,900 people, while the same number were flown out on 35 coalition aircraft.
The law ordering government use of civilian airliners has only been invoked twice - to fly troops for the 1990-91 Gulf War and again in 2002-2003 for the Iraq invasion.
READ MORE: British military: Seven Afghan civilians killed in Kabul airport chaos