Boris Johnson vows to rescue British Nationals as Afghanistan crumbles
With President Ashraf Ghani fled, and insurgent fighters surrounding the capital, the Prime Minister said the situation was "extremely difficult".
After chairing a meeting of the Government's Cobra contingencies committee he said the UK was determined to work with allies to prevent the country again becoming a "breeding ground for terror".
However he faced a backlash from MPs who said the West had been humiliated by insurgents armed with just basic weaponry.
MPs are expected to to vent their anger and frustration when they return to Westminster on Wednesday for an emergency recall of Parliament to discuss the crisis.
In the meantime, Mr Johnson said the Government's priority was to assist the remaining British nationals as well as those Afghans who had helped the UK.
He said the British ambassador Sir Laurie Bristow had been at Kabul airport helping to process the applications of those seeking to leave.
"Our priority is to make sure that we deliver on our obligations to UK nationals in Afghanistan, to all those who helped the British effort in Afghanistan over 20 years, and to get them out as fast we can," he said.
"We are going to get as many as we can out in the next few days."
Britain has sent 600 troops - including Paras from 16 Air Assault Brigade - to assist in the operation.
Meanwhile other Western countries were scrambling to get their people out, with helicopters shuttling from the US embassy to the airport while smoke was seen coming from the embassy rooftop as diplomats burned sensitive material.
With a new Taliban-led Afghan government expected to take power in a matter of days, or even hours, Mr Johnson said the UK would be working with allies to take a concerted approach to the new regime.
"We don't want anybody to bilaterally recognise the Taliban," he said.
"We want a united position among all the like-minded, as far as we can get one, so that we do whatever we can to prevent Afghanistan lapsing back into a breeding ground for terror."
However senior MPs expressed concern that the credibility of the West had been fundamentally damaged by its failure to support an ally 20 years after international forces first entered the country.
The chairman of the Commons Foreign Affairs Committee, Tom Tugendhat, said it was "the biggest single foreign policy disaster since Suez" in the 1950s.
The Defence Committee chairman Tobias Ellwood told Times Radio: "This is completely humiliating for the West.
"We assembled the most incredible, technologically advanced alliance the world has ever seen and we are being defeated by an insurgency that's armed with AK47s and RPGs."
Labour leader Sir Keir Starmer said the Prime Minister needed to set out plans to prevent the fall of the Afghan government turning into a humanitarian crisis, with thousands of displaced people trying to escape the Taliban.
There was particular concern about the plight of those Afghans who had worked with the UK and other Western countries, amid fears they would be targeted by the insurgents.
The Taliban insisted they were seeking a peaceful transfer of power and promised an amnesty for those who had worked with foreign countries or the Afghan government.
However such assurances were met with deep scepticism amid fears they would return to the hardline policies they pursued before they were forced out in 2001 - including the suppression of women and girls.
Mr Tugendhat told BBC News: "The real danger is that we are going to see every female MP murdered, we are going to see ministers strung up on street lamps."
Labour called for the urgent expansion of the scheme to re-settle Afghans who had worked with the UK.
Shadow foreign secretary Lisa Nandy said she had been inundated with appeals for help and that the Government had just hours to resolve the issue.
"Some of them have already been killed, others have received threats to themselves and their families.
"We have an obligation as a country to make sure that they are safe," she told BBC Radio 4's The World This Weekend.
The Home Office said it had already resettled 3,300 Afghan staff and their families and was continuing to fulfil its "international obligations and moral commitments".
Among senior parliamentarians there was shock at the speed of the Afghan collapse after the West had invested billions in building up the country's armed forces.
In the course of little over a week many cities fell to the Taliban without a fight after tribal elders stepped in to negotiate the withdrawal of government forces in order to avoid bloodshed.
While much of the anger was directed at the US for its decision to withdraw its forces, precipitating the collapse, some MPs expressed concern that Britain could have done more to avert the crisis.
Mr Johnson said however that while the US decision had "accelerated things", the end was inevitable.
"This has been in many ways something that has been a chronicle of an event foretold.
"We've known for a long time that this was the way things were going," he said.
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