Joe Biden has pledged to Americans still trapped in Afghanistan: “We will get you home.”

Mr Biden’s comments at a White House news conference early on Saturday morning come as the US government struggles to ramp up a massive airlift clearing Americans, other foreigners and vulnerable Afghans through the Kabul airport and rescuing them following a Taliban takeover of the country.

The US president is facing criticism for a chaotic and often violent scene outside the airport as crowds struggle to reach safety inside. He called the past week “heartbreaking,” but insisted his administration was working hard to smooth and speed the evacuations.

“I don’t think anyone of us can see these pictures and not feel that pain on a human level,” Mr Biden said, but “now I’m focused on getting this job done.”

As many as three flights out of Kabul were expected early on Saturday morning (Australian time), going to Bahrain and carrying perhaps 1,500 evacuees in all, said an official. Some 5,700 had been flown out overnight.

With desperate crowds thronging Kabul’s airport, and Taliban fighters ringing its perimeter, the US government renewed its advice to Americans and others that it could not guarantee safe passage for any of those desperately seeking seats on the planes inside.

The August 31 deadline “is contributing to the chaos and the panic at the airport because you have Afghans who think that they have 10 days to get out of this country or that door is closing forever,” said Republican Peter Meijer, who served in Iraq and also worked in Afghanistan to help aid workers provide humanitarian relief.

While Mr Biden has previously blamed Afghans for the US failure to get out more allies ahead of this month’s sudden Taliban takeover, US officials told The Associated Press that American diplomats had formally urged weeks ago that the administration ramp up evacuation efforts.

Mr Biden has said that the chaos that unfolded as part of the withdrawal was inevitable as the nearly 20-year war came to an end.

He said he was following the advice of Afghanistan’s US-backed president, Ashraf Ghani, in not expanding US efforts to fly out translators and other Afghans in danger for the past work with Americans any earlier. Mr Ghani fled the country last weekend as the Taliban seized the capital.

Mr Biden also said that many at-risk Afghan allies had not wanted to leave the country. But refugee groups point to years of backlogs of applications from thousands of those Afghans for visas that would let them take refuge in the United States.

Australia urged to take more refugees

Further flights into Kabul are being carefully planned as the government seeks to rescue hundreds of Australian citizens, former interpreters and embassy guards.

But the mission is being hampered by Australian military personnel being unable to go beyond the airport due to the Taliban’s chaotic and increasingly violent takeover of the Afghanistan capital.

“There are people in their thousands crowding around the entrances to the airport, and there have been – unfortunately – injuries as well and we have had to address some of those amongst our passenger cohorts, too,” Foreign Minister Marise Payne said.

“It is dangerous.”

It has been reported 12 people have been killed in and around the airport, where more than 5200 American troops have based themselves.

Prime Minister Scott Morrison said he had been advised sending Australian troops in to the city to help people was not viable.

“The United States continues to engage directly with the Taliban about the arrangements … enabling flights to go in and out of the airport,” he said.

“But we are dealing with the Taliban, so I’m not making any assumptions, and I am moving as quickly and as safely as we possibly can to get as many people out as fast as we can.”

More than 160 Australians and Afghan visa holders have now been evacuated from Kabul after a third rescue flight left the airport.

Sixty citizens and Afghans who helped Australia during the war were transported to the United Arab Emirates on Thursday night.

The first Australian flight from Dubai carrying 94 evacuees touched down in Perth in the early hours of Friday.

Mr Morrison thanked British counterpart Boris Johnson for the Royal Air Force’s assistance in a phone call after the operation.

Australia has defended allocating 3000 places this financial year in its humanitarian program to Afghans fleeing the Taliban.

Mr Morrison has described the figure as a floor rather than a ceiling.

Canada has committed to taking 20,000 refugees, while the UK will do the same but over five years.

Liberal MP John Alexander said Australia should accept refugees from Afghanistan over and above the annual refugee quota.

“We have a duty to provide sanctuary to Afghan refugees, as Australia played its own small part in the creation of the situation that we now see before us. It’s a matter of national honour,” Mr Alexander said.

-with AAP