Britain has begun a last-ditch scramble to get people out of Kabul amid warnings from the senior diplomat on the ground that staying past the 31 August deadline may not be realistic and risks provoking the Taliban.

Speaking to MPs from Kabul, Sir Laurie Bristow, the British ambassador to Afghanistan, said trying to hold Kabul’s airport any longer would be fraught with risk.

His remarks appear to put him at odds with Boris Johnson, who is due to lobby the US president, Joe Biden, at Tuesday’s G7 summit about the possibility of extending the evacuation beyond the end of the month.

But in a frank admission about the dire situation in the Afghan capital, Bristow made clear the Taliban would not tolerate western forces staying into September – a spokesman for the group said on Monday this would cross a “red line” and “provoke a reaction”.

He said: “The signalling that we’re seeing from the Taliban, including earlier today, is pretty uncompromising that they want the operation finished by the end of the month.

“So I think it follows from that, that if the US and its allies were to try to push beyond that, then there’s at least a risk there, of us doing so in a much more difficult and less compliant environment.”

The virtual meeting also heard that planning by the British military for the end of the emergency airlift by the RAF, which has evacuated more than 5,700 people since 13 August, had already begun.

Maj Gen Nick Borton, the chief of staff for operations, said they were “now starting to plan the conclusion” of the evacuation “and the difficult business of drawing the operation to a close” eight days before the current deadline.

A final decision by the US is likely to emerge from Tuesday’s virtual G7 meeting, but any agreement would have to be negotiated with the Taliban, who control Kabul, the airport perimeter and access to it.

Officials in Washington confirmed that the UK has asked for the deadline for the Kabul evacuation to be extended, and the US has been resistant.

“We would absolutely consider the views and opinions of our allies and partners who also have people there, and are .. very much a part of moving people out,” said John Kirby, the Pentagon spokesperson.

A source familiar with the discussions said: “Biden’s stance is different from the UK stance on that, and I think there’s a recognition that the 31st makes it very difficult to get things done in time, so that’s being explored.”

“The challenge is of course the Taliban no doubt will have a view on that, so there’s a question about how much flexibility there actually is for decision making,” the source added.

Johnson and Biden spoke on Monday evening ahead of the G7 meeting, the first time they had spoken for nearly a week. The two leaders discussed the progress of the emergency airlift, according to statements from the White House and Downing St.

“The leaders agreed to continue working together to ensure those who are eligible to leave are able to, including after the initial phase of the evacuation has ended,” Downing St added. “The Prime Minister and President Biden noted the importance of concerted diplomatic engagement to secure the progress made in Afghanistan and prevent a humanitarian crisis.”

The warning from Bristow came as part of the first briefing of MPs by the government since the start of the Afghan crisis a couple of weeks ago.

The briefing also heard that:

The threat to British troops and evacuees in Kabul from Isis’s affiliate in Afghanistan is deemed to be “severe”. Many of its members have been released from prison by the Taliban as they gradually took control of the country. On Monday morning there was a firefight at Kabul airport during which an Afghan guard was killed.

A warning that six people deemed “a direct threat” to the UK had presented themselves to British authorities handling resettlement claims.

Diplomatic and immigration staff based in Kabul were “getting burned out quite fast”, according to Bristow, because of the pressure of dealing with people wanting to be evacuated. Overnight, five Foreign Office staff joined the diplomatic team to help give those on the ground a break.

MPs have complained individual cases were getting lost in the system. Lisa Nandy, the shadow foreign secretary, said the “consular hotline just isn’t working” and presented a list of questions she hoped ministers would answer. Chris Bryant, a Labour backbencher, added: “We’ve got caseworkers who are in tears because they’re just not getting any help anywhere.”

Ministers had already begun hinting publicly that an end to the airlift could be close. Ben Wallace, the defence secretary, said the evacuation was “down to hours, not weeks“.

The junior defence minister James Heappey said the focus was now on evacuating about 1,800 British passport-holders, and 2,275 people already accepted as having a right to resettle in the UK because they had helped as interpreters.

Kabul airport is held by more than 5,000 US troops with the support of 1,000 from the UK, as well as forces from other countries – but without the US presence and the effective permission of the Taliban it cannot remain open.

A spokesperson for the Taliban, Suhail Shaheen, told Sky News there could be a military response if Washington were to extend its evacuation operation. He said: “If they are intent on continuing the occupation, so it will provoke a reaction.”

Borton also told MPs that the British military had expected to complete an airlift in a Kabul that was still held by Afghan security forces because most predictions were that the capital would not fall until later in the year.

Plans were also drawn up in the event that Kabul was held by a hostile Taliban, but Borton said that what nobody had anticipated was the the evacuation would be conducted in a “peaceful Kabul under Taliban control”.

Communication between the White House and Downing Street during the crisis appears poor, with British ministers and officials repeatedly complaining that the US administration has not been clear with allies how long it plans to keep its military on the ground, and allow the evacuation to continue.

On Sunday, the US secretary of state, Tony Blinken, acknowledged that it had been a “powerfully emotional time for a lot of allies and partners”.

“I’ve heard, across the board, deep appreciation and thanks from allies and partners for everything that we’ve done to bring our allies and partners out of harm’s way,” he told Fox News.