Prime Minister Boris Johnson on Wednesday said Britain owed "an immense debt" to Afghans who worked with NATO forces as he announced "vital support" for those resettling in the UK.

But his government is coming under fire after thousands of Afghans who helped NATO and are eligible to move to Britain under the "Relocations and Assistance Policy" were believed to be have been left stranded in Afghanistan, where they are at the mercy of the Taliban.

More than 8,000 did make it out, and the government announced on Wednesday that they will be given immediate indefinite leave to remain and that £15 million would be provided for additional school places and to support access to the health service.

"We owe an immense debt to those who worked with the Armed Forces in Afghanistan and I am determined that we give them and their families the support they need to rebuild their lives here in the UK," Johnson said of the so-called "Operation Warm Welcome" measures.

"I know this will be an incredibly daunting time, but I hope they will take heart from the wave of support and generosity already expressed by the British public."

The government said the measures "will give Afghans the certainty and stability to rebuild their lives with unrestricted rights to work and the option to apply for British citizenship in the future."

But current and former officials have condemned the government, suggesting many more could have been rescued. 

The Observer on Sunday cited a whistleblower as saying thousands of emails from MPs and charities to the foreign ministry highlighting specific Afghans at risk from the Taliban takeover went unopened.

Foreign Minister Dominic Raab has already been strongly criticised for not immediately leaving a beach holiday when the Taliban took control.

An unnamed minister also told the Sunday Times that "I suspect we could have taken out 800-1,000 more people".

Raab faces a tough hearing before the Foreign Affairs Committee on Wednesday, when he will be grilled by MPs over the chaotic withdrawal.

Announcing the evidence session, Chair of the Foreign Affairs Committee Tom Tugendhat said that "the last two weeks have been tough for so many of us: anger, shame, even incredulity". 

"We never thought we would see the day NATO forces, led by the US, would turn their backs on the people of Afghanistan. 

"How will we deal with the Taliban? How will Afghanistan shape our regional strategy? How will the government hold the Taliban to account for reverses in human rights?

"These questions, and so many others, will be put to the Foreign Secretary."

Britain in talks with Taliban

Britain has opened talks with the Taliban over the "safe passage" of its remaining nationals and allies out of Afghanistan after the hardline Islamist group's quickfire takeover of the country last month.

The British government confirmed to AFP it has dispatched senior civil servant Simon Gass to meet with Taliban representatives in Doha.

Much of the group's senior leadership lived in exile in the Qatari capital until the overthrow of Afghanistan's Western-backed government after 20 years of war.

Prime Minister Boris Johnson has come under fire after many Afghans who helped NATO -- and are eligible to move to Britain -- were believed to have been left stranded in Afghanistan, where they are at the mercy of the Taliban.

Gass is "meeting with senior Taliban representatives to underline the importance of safe passage out of Afghanistan for British nationals, and those Afghans who have worked with us," a government spokesperson told AFP on Wednesday.

It is the first publicly disclosed diplomacy between London and the Taliban since Britain joined the United States in the mammoth airlift of more than 100,000 people out of the country after the Afghan military's capitulation. 

The Taliban has pledged to allow Afghans to come and go in the face of calls from the international community to honour that commitment in the days after the US withdrawal on Tuesday. 

More than 8,000 Afghans who helped NATO forces made it out of Afghanistan and the British government said they would be given indefinite leave to remain.

But criticism was levelled at the government for not evacuating hundreds more stuck in the war-torn country as the Taliban took control.

British Foreign Minister Dominic Raab was also condemned by the opposition Labour party for not immediately leaving a beach holiday when the Taliban took control.

An unnamed British minister told the Sunday Times that he believed the UK could have evacuated "800-1,000 more people" in the chaotic airlift.

Johnson's government sought to extend the US withdrawal deadline of August 31 but ultimately failed to persuade President Joe Biden.

After the Taliban swept into Kabul in mid-August, the British premier said the Taliban must be judged on its "actions rather than by its words" and insisted Britain could not have remained in Afghanistan without American support.