Addressing parliament, Prime Minister Boris Johnson failed to answer a question on how many British-Afghans remained after a hurried Western airlift was ended.

Thousands made it out from Kabul on packed Royal Air Force planes, but he said 311 people were left behind who are eligible for the UK's Afghan Relocations and Assistance Policy, such as interpreters.

"We will do everything we can to ensure that those people get the safe passage that they deserve," Johnson said.

The airlift was "one of the most spectacular operations in our country's post-war history", he said, vowing an "equal effort" to house and educate Afghans newly arrived in Britain.

Johnson demanded the Taliban honour commitments to allow out those wishing to leave, and to respect women's rights, if the militia wants to gain access to billions of dollars in Afghan funds frozen overseas.

Speaking ahead of Saturday's 20th anniversary of the 9/11 attacks by Al-Qaeda, the prime minister pressed the Taliban in particular to prevent Afghanistan becoming a haven for extremists to launch attacks abroad.

Johnson defended the UK and Western intervention in 2001 against critics, including in his Conservative party, who say the Taliban's triumphant return to power shows the effort was in vain.

He said that later this month, he would press at the UN General Assembly in New York for consensus to hold the Islamists to account.

"We will judge the Taliban by their actions, not their words -- and use every economic, political and diplomatic lever to protect our own countries from harm and to help the Afghan people," the UK leader said.