President Joe Biden has told US citizens stuck in Afghanistan and allies who worked alongside American forces “we will get you home”, but admitted he cannot promise what the “final outcome” of the troop withdrawal will be.

In a news conference at the White House, he admitted images of people desperately trying to flee the Taliban at the airport in Kabul were “heart-breaking” and “gut-wrenching”, but insisted he is “focused on getting this job done”.

He said: “This is one of the largest, most difficult airlifts in history. We’re going to do everything that we can to provide safe evacuation to our Afghan allies and partners.

“Any American that wants to come home, we will get you home.”

He added: “It’s time to end this war.”

Mr Biden said 13,000 people had been safely evacuated from Afghanistan since 14 August – the day before Kabul fell to the Taliban – but added the mission was “dangerous” and “every resource necessary” would be mobilised, with nearly 6,000 troops currently on the ground.

He said the US would “retain a laser focus” on its counter terrorism mission in the country to prevent it being used as a base to launch attacks, and warned the Taliban that any threat to the US forces would be met with a “swift response”.

Mr Biden has faced a torrent of criticism for the rapid withdrawal of US troops from Afghanistan, with the Taliban now back in control of the country 20 years after being ousted.

He rejected the condemnation, saying the pull-out of military personnel was the “only rational thing to do” and insisted that US allies agreed with the decision.

Speaking to reporters, he said: “I’ve seen no questioning of our credibility from our allies around the world,” and he said: “I’ve not seen that, as a matter of fact the exact opposite.”

He added: “There’ll be plenty of time to criticise and second guess when this job is done.”

There is a race against time to get westerners and their Afghan allies, including translators, interpreters, and other vulnerable people, out as American soldiers, who are leading the evacuation effort, are due to leave the country by 31 August.

Tens of thousands of people remain to be evacuated ahead of the deadline.

There have been chaotic scenes outside Kabul’s airport every day this week as desperate crowds of Afghans try to flee the country.

Thousands are thronging the facility as they try to escape the Taliban but some face difficulties even getting to that point due to armed checkpoints by the group’s fighters.

The president said he had “no indication” that US citizens were being blocked by the militants from reaching the airport.

Some US politicians are calling on the Biden administration to extend a security perimeter beyond the airport and further into Kabul so more people can make it to the airport for evacuation.

But the president rejected this suggestion, saying it could have “unintended consequences”, indicating there could be additional security threats against US troops. But he added “judgements” will be made by commanders on the ground.

Some in Congress also want Mr Biden to make clear the 31 August deadline for withdrawing US troops is not a firm one.

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Matt Zeller, who served in Afghanistan and is a former combat adviser to the Afghan security forces, said he had told US officials back in February that they should start evacuating Afghans, and called the current situation a “nightmare scenario”.

He told Sky News: “Imagine if we had done this back in the winter when it wasn’t fighting season, when we still had forces in place and controlled every airfield in Afghanistan.

“Would it have been chaotic to begin moving people slowly, methodically, a couple of families at a time, when we had the personnel and equipment in place? Would that have been chaotic? Absolutely not. Could we have done it, probably quietly? You betcha.”

Mr Zeller added: “I cant think of something more horrific and a fiasco than what’s going on now.”

“We don’t have a good security perimeter. We don’t have enough runway space. I can’t imagine what it must be like to be a US marine standing metres away from a Taliban thug who is beating on US citizens.”

He said he had personal accounts of American citizens standing in line waving their passports but not being able to get in.

The US government had earlier renewed its advisory to Americans and others that it could not guarantee safe passage for any of those desperately seeking seats on the planes inside.

It said: “We are processing people at multiple gates. Due to large crowds and security concerns, gates may open or close without notice. Please use your best judgement and attempt to enter the airport at any gate that is open.”

A US defence official said around 5,700 people, including about 250 Americans, were flown out of Kabul aboard 16 C-17 transport planes in the last 24 hours.