Britain fears US forces may pull out of Kabul international airport within days, putting it at risk of closure and raising concerns over the emergency airlift of thousands of people from Afghanistan.

Whitehall and security sources said they could not guarantee how long the US would keep its contingent of 6,000 troops on the ground and cautioned that the UK could not continue the rescue without their presence. They also indicated Britain was not engaging with the Taliban directly over security or other issues after the militant group seized the Afghan capital.

The Guardian has learned that some in government, however, believe there is a shift by UK ministers and the military towards dealing directly with the Taliban and legitimising their role – a position that would anger those who believe they have not changed.

Gen Sir Nick Carter, the head of the British armed forces, said on Wednesday he thought the Taliban wanted an “inclusive Afghanistan” and described them as “country boys” who had “honour at the heart of what they do”. Asked on Sky News about the Taliban’s repression of women, Carter said: “I do think they have changed and recognise Afghanistan has evolved and the fundamental role women have played in that evolution.”

Boris Johnson also hinted at the possibility of recognising the Taliban, potentially in conjunction with other countries, telling MPs: “We will judge this regime based on the choices it makes and by its actions rather than by its words.”

A Whitehall source said uncertainty over the Taliban’s actions, however, and the US position, meant the UK wanted to complete its evacuation as rapidly as it could, saying: “There’s a realistic view that we want to just go as quickly as possible.”

The airport in Kabul was the scene of chaos this week but has since been secured by the US ahead of a planned evacuation deadline of 31 August. British attempts to seek reassurances from the US over that timeline had not proved successful, a source said, although on Tuesday the US national security adviser, Jake Sullivan, tweeted that the country would hold the airport “to get all Americans out of Afghanistan”.

A total of 700 Britons, Afghans and others were airlifted out of Kabul on Tuesday, according to official figures, taking the total to more than 1,150 out of as many as 6,000, half of which are Britons and dual nationals and the remainder Afghans eligible to settle in the UK because they previously helped the British. Of the 1,150, 300 are Britons.

Carter said he expected seven aircraft to head to Kabul, enabling up to a further 1,000 people to leave on Wednesday. “The situation has stabilised since the weekend but it remains precarious,” he said.

The operation of the airport is also dependent on the Taliban, who now control its surroundings. Military experts say it is easy to close an airport by firing mortars or shells on to the runway.

A particular problem is the difficulty of eligible people getting to the airport. Some Britons have been advised to say, at Taliban checkpoints, that they want to “go to the airport and leave the country”, which it is feared could put them at risk of reprisals.

A security source said the UK was having to rely on “uncomfortable intermediaries” in its dealings with the Taliban, who have a separate agreement with the US to allow it to conduct its retreat. Insiders also acknowledge the UK has limited intelligence on what is happening outside Kabul.

Carter’s interview with Sky News, which followed a Taliban press conference on Tuesday, prompted a furious reaction among MPs. The Conservative MP Nusrat Ghani told the Guardian: “Not a single Afghan woman has stated this naive optimism about the Taliban. They have not changed, women are hiding at home in fear of having been teachers and lawyers and just yesterday a women was killed in Afghanistan for not covering her hair.”

Caroline Nokes, the Tory chair of the equalities committee, said: “I would rather judge them by their actions than their words. Look at the pictures of the streets of Kabul, the women have disappeared, gone into hiding, especially those who have any sort of leadership role. The civic mayors, the activists, the journalists, the judges. The women we have encouraged to step up, to build civic society in Afghanistan, they are the ones most likely to suffer reprisals.”

The Labour MP Stella Creasy said: “Anyone thinking that Taliban pledges on women’s rights are enough – as if it’s equality being able to leave the house alone – needs to ask if they would be happy to live under such restrictions and feel an equal citizen.”

Asked about Carter’s position, Johnson’s spokesperson said: “He was reflecting what was claimed by the Taliban.”