Video report by ITV News Correspondent Dan Rivers
The UK has "hours, not weeks" to complete its evacuation of Britons and local allies from Afghanistan, the defence secretary has warned, with a "precarious" security situation at Kabul Airport.
Ben Wallace reiterated that "not everyone will get out" of Afghanistan even if they're legally entitled to enter the UK.
He told broadcasters the UK would be unable to unilaterally keep soldiers in Afghanistan past the August 31 deadline agreed by the US and Taliban because the huge support from America would cease.
Boris Johnson plans to lobby his US counterpart Joe Biden for an extension to the deadline but comments from the president that he hopes to stick with the agreement makes success seem unlikely.
It comes as a Taliban spokesman said there would be "consequences" if the agreement was broken.
The Ministry of Defence said on Monday: "British forces will remain in place to ensure those Afghans who are eligible for relocation to the United Kingdom that want to, are registered and evacuated as the UK accelerates the Afghan Relocation and Assistance Policy."
ITV News Global Security Editor Rohit Kachroo and Political Correspondent Shehab Khan explain the latest on the situation in Afghanistan
The news comes as the Pentagon announced 16,000 people had been evacuated from Kabul overnight.
The Pentagon also said overnight a member of the Afghan security forces was killed in a firefight.
Speaking to reporters in Fort George, near Inverness, Defence Secretary Wallace said: "The Prime Minister is, obviously at the G7, going to try and raise the prospect of seeing if the United States will extend.
"It's really important for people to understand the United States have over 6,000 people in Kabul airport and when they withdraw that will take away the framework... and we will have to go as well.
"I don't think there is any likelihood of staying on after the United States. If their timetable extends even by a day or two that will give us a day or two more to evacuate people.
"Because we are really down to hours now, not weeks, and we have to make sure we exploit every minute to get people out."
He added: "It is moving rapidly on the ground, if we get more days we get more people out... but then if there was a security concern and we had to seal the gate, suddenly you lose hours and if you lose hours you lose people."
Taliban spokesperson Dr Suhail Shaheen told Sky News that August 31 is "a red line".
"President Biden announced that on 31 August they would withdraw all their military forces. So if they extend it that means they are extending occupation while there is no need for that." He added: "If the US or UK were to seek additional time to continue evacuations - the answer is no. Or there would be consequences."
Defence Secretary Wallace admitted the Taliban now has a seat at the negotiating table as he accepted the need to continue working with the Islamist group on the ground in Kabul.
Asked if he believes the US will extend the deadline, he said he expects President Biden to "explore it, but I think we'll have to see what happens".
He added: "The United States and the G7 are not the only not the only stakeholders in this, the Taliban now have a vote and the security situation is precarious."
Chaotic scenes in Kabul have been blamed for hampering the evacuation effort, with thousands of ineligible Afghans descending on the city's airport, making it more difficult for others to get inside.
ITV News filmed with crowds of British citizens blocked from escaping Afghanistan, despite displaying UK passports, as chaos at Kabul Airport left those eligible for evacuation unable to get inside.
'Just get us out': Britons blocked from evacuation at Kabul Airport - watch ITV News Senior International Correspondent John Irvine's eyewitness report from outside the airport
Distressing images showed dozens of Afghans and Britons trying to enter a temporary British embassy but be turned away without reason, as Taliban fighters dispersed crowds with volleys of gunshots into the air.
Disorder at the airport was compounded when an Afghan security officer was killed amid a gunfight which broke out at the airport's north gate early on Monday morning, the German military said.
Three others were injured by the "unknown attackers," the German military said, before their own troops, as well as US troops, arrived.
The British military said on Sunday another seven people had been killed in the unceasing crush of crowds outside the airport.
Mr Wallace said: "The speed at which we need to leave, the challenges around crowds and many people who don't meet any criteria but are trying to get out of the country, creates real problems for us in making sure everyone gets out and I've been clear, not everyone will get out."
In a tweet, Mr Johnson it was “vital that the international community works together to ensure safe evacuations”.
Asked about delaying the withdrawal of American troops during a press conference, Mr Biden said: “Our hope is that we don’t have to extend but there are discussions going on about how far we are.”
No 10 said "discussions on the ground" had been held with the Taliban over extending the deadline evacuations, but officials were still working towards a deadline of the end of the month.
The Prime Minister's official spokesman said: "Given the Americans are providing the vast bulk of security and facilitating the functioning of the airport in Kabul, and given we have previously spoken to other Nato countries about staying and that didn't prove to be a viable option, we are going to need to leave the airport at the same time as the Americans."
He added that although the Government was working towards an August 31 deadline, he said: "No firm date has been set in terms of when our evacuation procedure ends
Biden: Pulling out of Afghanistan would always create 'pain' and loss'
Labour leader Sir Keir Starmer has written to the PM 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 President 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”.
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 Monday morning, defence minister James Heappey said than 1,800 people had been evacuated in the past 24 hours.
Brigadier Dan Blanchford, the most senior UK military officer on the ground in Kabul, said British troops had “witnessed some harrowing scenes”, with at least seven Afghan civilians confirmed to have died outside the airfield gates amid chaotic crowds.
It has been reported that as many as 20 people have been killed in the past week while trying to get into the departure point.
Brigadier Blanchford, Commander Joint Forces Operations, said: “The horrific difficulties which families and individuals have in getting to the airport are clear and my men and women on the frontline have seen and witnessed some harrowing scenes.”
Brigadier Blanchford added he was “proud” of the work of the armed forces in Kabul “who have shown professionalism and compassion in challenging circumstances”.
He said UK armed forces were “redoubling” efforts to “speed up the process” of helping people make their exit, with the Times reporting that the RAF hopes to fly out as many as 2,000 people per day.