The Taliban has said there will be no evacuations from Afghanistan after the August 31 deadline as Boris Johnson was expected to urge the US to keep troops in the country past the end of the month to allow more people to escape.
The militants said they will take a “different stance” after the cut-off date for evacuations passes.
And during a press conference in Kabul spokesman Zabihullah Mujahid said: “August 31 is the time given and after that it’s something that is against the agreement.
“All people should be removed prior to that date.
“After that we do not allow them, it will not be allowed in our country, we will take a different stance.
An extension to the deadline for pulling out remaining troops from Afghanistan to allow evacuations to continue is “unlikely”, Defence Secretary Ben Wallace earlier conceded.
But at a virtual G7 summit on Tuesday afternoon Boris Johnson was expected to press US President Joe Biden to keep his troops in the Afghan capital for longer.
The Taliban, which swept to power last week in the wake of America’s major withdrawal of troops, has suggested that foreign forces remaining past the deadline would cross a “red line” that will “provoke a reaction”.
The group warned on Tuesday that evacuations “will not be allowed” after August 31.
And a spokesman said the Afghans should “return to their homes and resume their calm everyday lives”.
Mr Mujahid said crowding at the airport was dangerous and “people could lose their lives”.
And he urged the US not to “encourage” highly skilled people to leave Afghanistan.
Earlier, Mr Wallace said it was not just what the Taliban had said that made him doubt the extension would be granted, but also the attitude in the US.
“I think it is unlikely. Not only because of what the Taliban has said but if you look at the public statements of President Biden I think it is unlikely,” he told Sky News.
“It is definitely worth us all trying and we will.”
Britain has evacuated 8,600 people from Afghanistan in the past 10 days, including more than 2,000 in the previous 24 hours, according to the Defence Secretary.
Mr Wallace has insisted it would not be appropriate to try to secure Kabul airport with British troops after the US pulls out.
“It’s not about effectively whether I could fly in thousands of troops and secure the airport,” he told BBC Radio 4’s Today programme.
“Yes I could do that, I could probably secure the airport for a few months, or maybe a year or two.
“But for what purpose? For them to be shot at, attacked, people not to get to the airport and to trigger just a permanent fight? I don’t think that is a solution.”
Mr Wallace envisaged the “consequences” threatened by the Taliban of missing the deadline to leave could range from preventing people going to Kabul airport to “military activity that could potentially close the airport”.
The Prime Minister spoke to Mr Biden on Monday evening ahead of the G7 leaders’ call, which will also include the general secretaries of Nato and the United Nations.
“The Taliban will be judged by their deeds and not their words,” the Prime Minister added.
UN human rights chief Michelle Bachelet warned she had already received credible reports of “summary executions” and restrictions being imposed on women on Taliban-controlled areas.
Mr Johnson has promised “to use every humanitarian and diplomatic lever” to protect human rights in Afghanistan.
Leaders of the G7, which also includes Canada, France, Germany, Italy and Japan, are further expected to discuss aid and the burgeoning refugee and humanitarian crises.
Ahead of the summit, six chairs of foreign affairs committees in G7 parliaments issued a joint statement urging against “arbitrary dates” for ending military support for the evacuation.
Signed by Bob Menendez, a US senator in Mr Biden’s Democratic Party, and the senior Tory MP Tom Tugendhat, they also urged against “artificial caps on the number of evacuees”.
Sir Mark Lyall Grant, a former national security adviser, said it was “totally unrealistic” to think there could be an extension to the deadline without the Taliban’s consent.
“The Taliban could stop this evacuation process in one hour’s time if they started firing missiles at departing planes, if they lobbed a mortar into the airport, so this evacuation is happening with the cooperation of the Taliban,” he told BBC Radio 4’s World at One.