Boris Johnson is expected to urge Joe Biden to extend the Kabul evacuation deadline as ministers conceded that the Taliban would also have to agree to more time for people to flee.
Armed forces minister James Heappey acknowledged that “when the US go, the mission has to come to an end” in Afghanistan as the Prime Minister prepares to issue the plea to the American president.
The leaders will speak during an emergency G7 summit on Tuesday as the Government presses for American troops to remain beyond August 31 to secure the capital’s airport for rescue flights.
Ministers still want to evacuate thousands more people including UK passport holders and those under the Afghan relocations and assistance policy (Arap) scheme.
Mr Heappey said the evacuation mission is “fundamentally underpinned by a US presence”, and it would have to end without American troops.
“Whether or not the US can be persuaded to stay is a matter for the Prime Minister tomorrow in the G7 meeting after the initial overtures made by both the Foreign Secretary and the Defence Secretary in the days previous,” he told Sky News.
“But the conversation with the Taliban will then follow, and the Taliban will have a choice: they can either seek to engage with the international community and show that they want to be a part of the international system, they want to be engaged in international diplomacy, or they can turn around and say there is no opportunity for an extension.
“I think everybody has to be clear that this is not just a discussion that happens between G7 leaders tomorrow, it is a discussion which happens with the Taliban.”
He acknowledged not everybody will be able to be evacuated, as he said there are still “thousands more” people the UK wishes to evacuate, including British nationals.
The minister was speaking as it was reported that a firefight at one of the gates of Kabul’s international airport killed at least one Afghan soldier on Monday.
The Foreign Office said it had sent five extra staff to Kabul airport, taking its total working on the evacuation effort in the capital to 19.
Mr Heappey elaborated to BBC Radio 4’s Today programme by saying that the Taliban, which swept to power last week as the US withdrew its troops, “gets a vote” on the evacuation deadline.
“It’s just the reality, we could deny them the vote, we have the military power to just stay there by force, but I don’t know that the humanitarian mission we’re embarked on at the moment which is to evacuate as many people from Kabul as we possibly can is helped by Kabul becoming a warzone,” the minister said.
He added: “I think in all reality given what Nato allies have in country at the moment, the period of time it would take to get in place a replacement force is not realistic, I think the reality is that the die is cast, the United States air force is operating Kabul airport, it is entirely a military airport.
“When the US go, the mission has to come to an end.”
Mr Biden signalled on Sunday that he did not want US armed forces to stay in the central Asian country beyond August.
Asked about delaying the withdrawal of American troops during a press conference, the president said: “Our hope is that we don’t have to extend but there are discussions going on about how far we are.”
Foreign Secretary Dominic Raab and Defence Secretary Ben Wallace held talks with their Washington counterparts over the weekend to call for an extension.
Labour leader Sir Keir Starmer has written to the Prime Minister calling for more information on how the UK is planning for the next stages of the rescue mission.
Sir Keir asked whether Mr Johnson had “spoken personally” to Mr Biden to “ask him to extend the evacuation period beyond the end of August”, and whether the UK was working on a contingency plan with Nato allies to “hold Kabul airport without US troops”.
Government officials said there is “no fixed date” on when the UK will withdraw, but it is feared that without US boots on the ground, the remaining allied forces would be unable to secure Hamid Karzai International Airport from the crowds looking to flee the Taliban takeover, or other potential security threats.
The Ministry of Defence confirmed that 5,725 people have been repatriated since rescue efforts began on August 13, with 3,100 of them Afghan individuals and their families.
On Sunday, 1,721 people were airlifted from Kabul by the Royal Air Force across eight flights.