The Afghan army was routed in a rapid Taliban advance this week, 20 years after a US-led invasion ousted the hardline Islamist group from power.

The Taliban's return has plunged Afghans who aided Australian troops during the occupation into grave danger, with reprisals against those who opposed the group already under way.

Speaking directly to Australian soldiers on Tuesday, Prime Minister Scott Morrison admitted some would not be saved despite Australia's last-ditch rescue attempts.

"Despite our best efforts, I know that support won't reach all that it should," he said.

"On-the-ground events have overtaken many efforts. We wish it were different."

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He accepted the Taliban's return would be a "day of sadness and reflection" for Afghanistan veterans.

"It's a time of deep and uncomfortable questioning, and that is only right. We shouldn't hide from it," he said.

"The scenes from Kabul have been absolutely heartbreaking. It's a sobering day for everyone, particularly those who have given so much over the past 20 years, and most notably those 41 who were lost."

But Labor MP and veteran Luke Gosling said veterans' pleas had fallen on deaf ears for months, with many devastated by the imminent death of people they worked alongside.

"For the Prime Minister to try to say he understands the hurt that it's causing the veterans community is just not believable," he told The Canberra Times.

"If he was concerned, he would have listened to the veterans that were coming to him and his MPs with the details of those that had served alongside Australians."

Mr Gosling said while the focus should saving lives for now, parliament should eventually consider an inquiry into the response.

"The Prime Minister's inaction means that Afghans, who are security vetted [and] did serve loyally with Australian troops, are now being hunted by the Taliban," he said.

"The federal government has known about this for months, and has failed to act. That's to their shame."

But Mr Morrison insisted 430 Afghan "locally engaged employees" had been repatriated since April, and 1800 had been moved to Australia in total.

"There will be more that will be added to this number, not just through the locally-engaged program that we have been running with a great sense of urgency," he said.

"Our humanitarian program more broadly ... has the capacity to ensure that we can also take people through those channels, in a very torrid situation like we have in Afghanistan at the moment."

About 250 Australian troops have been placed on standby to help the rescue efforts, but Mr Morrison dismissed a question over whether any were linked to the Brereton report as a "slur".

The Taliban has also begun targeting prominent female professionals and activists, along with members of the LGBT community.

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The federal government has confirmed no Afghan visa holder currently in Australia will be returned while the security situation remained volatile, but Mr Morrison declined to answer when pressed on whether they would be granted permanent residency.

The Prime Minister insisted the US's decision to withdraw from Afghanistan made Australia's presence no longer feasible.

"The sad truth is, with the decisions that have been taken, that is not something that Australia can continue to do," he said.

"I can say that Australia has done all within her power to get safety for the people of Afghanistan."

But Mr Morrison claimed to be optimistic flights would be able to return to Kabul, despite distressing footage showing thousands of Afghans scrambling to board a US military plane departing the city's airport on Monday.

At least two were later filmed clinging to the side of the aircraft before plunging to their deaths after takeoff.

Defence Minister Peter Dutton on Tuesday said the situation in Afghanistan was too dangerous for Australian forces to rescue their countrymen, numbering over 130, for now.

He confirmed air force troops would be positioned at a base in the UAE while the situation calmed.

In a televised address on Monday, a defiant US President Joe Biden conceded the speed of the Afghan government's collapse had taken Washington by surprise.

But he defended the withdrawal, arguing American troops should not be left to prop up the Afghan government indefinitely.

"I stand squarely behind my decision. After 20 years I've learned the hard way that there was never a good time to withdraw US forces. That's why we're still there," he said.

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