UK troops in Afghanistan have said they are facing challenges “nobody has experienced before”, as a former defence chief warned the Kabul airport evacuation is the “worst possible scenario”.

The Ministry of Defence (MoD) confirmed on Sunday that seven Afghan civilians had died in the chaotic crowds outside the airport, while reports have suggested as many 20 people have been killed in the past week while trying to get into the airfield.

The Operation Pitting rescue mission is being supported by 1,000 British troops – including Paras from 16 Air Assault Brigade – with a further 1,700 people repatriated in the past 24 hours.

Ministers are desperate for US President Joe Biden to keep American forces on the ground beyond his August 31 deadline, with Labour calling for the Prime Minister to phone the UK ally to extend the exit date.

Boris Johnson confirmed on social media that he will convene G7 leaders, which will include Mr Biden, on Tuesday for “urgent talks” on the situation in Afghanistan.

In a statement, White House press secretary Jen Psaki said the group would discuss “continuing our close co-ordination on Afghanistan policy” as well as ongoing evacuation efforts.

Involved in that exercise is Lieutenant Colonel Justin Baker from 16 Air Assault Brigade, who is stationed at the passenger handling facility at Hamid Karzai airport.

He said: “I think this is something that nobody has really experienced before, so we are facing challenges that nobody has experience for.”

Former chief of the defence staff Lord Richards said UK forces securing the air strip are in a “very precarious situation”, with little protection from security threats posed by the likes of so-called Islamic State.

“I’ve done a few of these non-combat evacuation operations in my time and this is the worst possible scenario, when you are unable to clear a space between the airhead and your possible threat, which is exactly what is happening now,” he told Times Radio.

“They are right on top of people and we’ve seen the tragic scenes coming out of Kabul airport, so it is not a good situation.”

Armed forces minister James Heappey said British troops are “seeing things that are unimaginably hard to deal with”, with reports of overcrowding and violence in the Kabul heat.

However, he said there have been “very encouraging” improvements at the Baron Hotel in Kabul, where Britons and Afghans eligible for repatriation are being processed, following reports earlier this weekend that it had been blockaded by the Taliban.

He said the “flow” outside the hotel is “significantly” better following changes to crowd control by the militant regime, with 731 people processed for UK flights on Sunday morning alone.

But the Afghan veteran echoed Defence Secretary Ben Wallace’s prediction that “absolutely nobody can say we’ll be able to get everybody out” as the UK presses for more time.

“The hard reality of complex, chaotic, dangerous situations like this is that simply may not be the case,” Mr Heappey added.

He confirmed Foreign Secretary Dominic Raab – who is once again in the spotlight over his decision to remain on holiday as the Taliban advanced – has spoken to his opposite number, US Secretary of State Antony Blinken, along with others in Washington to discuss extending the end-of-the-month deadline.

The US decision to pull out of Afghanistan has seen senior politicians and security experts call for a “rethink” on British foreign policy.

In an article for his website, former prime minister Tony Blair accused Mr Biden of engaging in an “imbecilic policy” after stating the US would exit Afghanistan to end “forever wars”.

Mr Blair, speaking to Sky News on Sunday, said he has “enormous respect” for the president, but he repeated his belief that the retreat “was not something we needed to do”.

Tobias Ellwood, Conservative chairman of the Commons Defence Committee, said suggestions that Britain had “not been included in the conversations” on the Afghan exit strategy shows there has been a “demise” in the “special relationship” between the UK and US.

Former national security adviser Lord Ricketts said the unfolding of events in the central Asian country had been “a humiliating period for the UK”.

The peer told Times Radio: “We need to rethink a lot of that rhetoric in the integrated review published by the Government a few months ago about Britain as an ‘independent sovereign operator, turning the dial on international crises’.

“We have shown actually that we are pretty impotent in a situation where the Americans take a decision – we have little choice but to follow.”

Meanwhile, the Foreign Secretary is facing further revelations about his decision to remain on holiday in Crete while the Taliban circled Kabul, with claims he stayed abroad despite being told to return to the UK.

The Sunday Times reported that Mr Raab was ordered home from his trip by Downing Street on August 13 but stayed for two more days at his five-star hotel on the Greek island because Mr Johnson told him he could.

Lord Ricketts said it is “very surprising” the Cabinet minister had chosen to stay abroad, while Labour deputy leader Angela Rayner called the alleged decision “a joke”.