The US has an “unwavering commitment” to get its citizens and at-risk Afghans to safety out of Afghanistan, President Joe Biden says, as Taliban fighters beat back thousands desperate to flee outside Kabul airport.

Mr Biden said the security situation in Afghanistan was changing rapidly and his administration was under no illusions about the threat from Islamic State militants in Afghanistan known as ISIS-K (for Khorasan).

The Taliban, which seized power in Afghanistan last week as the US and its allies withdrew troops after a 20-year war, fired in the air and used batons to force people to form queues outside the airport, witnesses said. On Saturday, seven people were killed in a crush at the gates.

“Let me be clear, the evacuation of thousands from Kabul is going to be hard and painful” and would have been “no matter when it began”, Mr Biden said in a briefing at the White House on Sunday (US time).

“We have a long way to go and a lot could still go wrong,” he said.

Australia’s evacuation missions have continued, with more than 470 people taken out of Kabul overnight on several RAAF flights.

Home Affairs Minister Karen Andrews said the most recent flight included Australians, Afghan nationals and British citizens. They were taken to a nearby military base in Dubai to be processed.

More than 800 people have now been evacuated from Kabul airport by Australia since the capital fell to the Taliban. Ms Andrews said the number who still needed rescued was changing constantly.

“There is a significant number. I don’t want to put a figure on it because it is changing almost by the hour as more people are seeking to come to Australia,” she said on Monday.

Mr Biden said he had directed the US State Department to contact Americans stranded in Afghanistan by phone, email and other means, and the US had a plan to move them to the airport.

“We’re executing a plan to move groups of these Americans to safety and to safely and effectively move them to the airport compound. For security reasons, I’m not going to go into detail … but I will say again today what I’ve said before: any American who wants to get home will get home.”

Afghan allies of the West and vulnerable Afghans such as women activists and journalists would be helped too, he said.

On Sunday, there were no major injuries as gunmen beat back the crowds, according to witnesses. The Taliban had been “co-operative” about extending the airport perimeter, Mr Biden said.

Asked if the US would extend an August 31 deadline for evacuations, Mr Biden said: “Our hope is we will not have to extend but there are going to be discussions I suspect on how far along we are in the process.”

The US has enlisted the help of six commercial airlines to help transport people from temporary locations after their evacuation from Afghanistan.

Britain’s defence ministry said seven Afghans were killed in the crush around the airport on Saturday as thousands tried to get a flight out. Sky News showed soldiers on a wall on Saturday attempting to pull the injured from the crush and spraying people with a hose to prevent them from getting dehydrated.

A NATO official said at least 20 people have died in the past seven days in and around the airport. Some were shot and others died in stampedes, witnesses have said.

British Prime Minister Boris Johnson called a virtual meeting of leaders of the Group of Seven wealthy nations for Tuesday to “ensure safe evacuations, prevent a humanitarian crisis and support the Afghan people”.

Britain plans to push world leaders to consider new sanctions on the Taliban when the G7 meets, sources told Reuters. Mr Biden said he would support that effort, depending on the conduct of the Taliban.

Panicked Afghans have tried to board flights out of Kabul since last weekend, fearing reprisals and a return to a harsh version of Islamic law the Sunni Muslim group exercised while in power two decades ago.

Leaders of the Taliban, who have sought to show a more moderate face since capturing Kabul, have begun talks on forming a government.

They face opposition from forces in northern Afghanistan, which said this weekend they had taken three districts close to the Panjshir valley.

Anti-Taliban leader Ahmad Massoud said on Sunday he hoped to hold peaceful talks with the Islamist movement but that his forces in the Panjshir were ready to fight.

-with AAP