The Taliban spokesman could not have been clearer. The August 31 deadline for foreign troops leaving Kabul airport is “a red line”, Dr Suhail Shaheen said, warning that if the US or the UK sought an extension, “there would be consequences”.

It is a crushing blow to those hoping to get thousands more vulnerable people out of Taliban-ruled Afghanistan. But ignoring the ultimatum is not an option.

The troops holding Kabul airport – including Britain’s elite 16 Air Assault Brigade and the US 82nd airborne – are among the most capable forces the Western world has to offer.

But they are in an impossible tactical position. The Taliban control the city of Kabul and all the surrounding countryside, including the commanding mountain heights overlooking the airfield.

US President Joe Biden is not about to airlift more US troops into Kabul to push the Taliban out of shelling range of the runway and hold the city indefinitely – in other words, restart the war – so use of force is off the table.

But what about talking?

Could the Taliban be persuaded that it would be in everyone’s interests to give the evacuation just a few extra days?

An absent military force, what leverage does the West have left?

Much has been made of Afghanistan’s dependence on international financial assistance – and the Taliban’s need to deliver a functioning government in order to maintain their tenuous grip on power.

Some three quarters of the Afghan national budget under the last government was paid for by foreign aid, and the collapse of the state has put the country on the brink of an economic crisis that could easily spiral into a humanitarian one.

Some Western officials clearly see that as a potential level of influence.

British Prime Minister Boris Johnson is expected to push the G7 leaders today to consider economic sanctions and withholding aid if the Taliban commit human rights abuses and allow their territory to be used as a haven for militants.

And pressure is already being applied.

The US has frozen nearly $9.5bn (€8bn) in assets belonging to the Afghan central bank and stopped shipments of cash to the nation.

Germany and Finland are halting all development aid to Afghanistan. Germany was one of Afghanistan’s biggest aid donors and had planned to spend nearly $300m in the country this year.

The Taliban’s relative co-operation with the evacuation effort so far will fuel optimism it can be bargained with.

But the carrot-and-stick strategy could backfire spectacularly.

The threat of “cutting the country off at the knees” is unlikely to produce the concessions the West seeks, warned Rory Stewart, a former UK international development secretary.

“My instinct is that kind of leverage never works. It hurts innocent people on the ground.

"It makes it impossible to keep the water supply going, the electricity going, the clinics going, the schools going, without having any impact at all on the leadership,” he said.

The West has few credible options for challenging the Taliban’s “red line”. So the best bet may be to end the military presence, and persuade the group to allow Kabul airport to resume civilian flights instead, perhaps with Turkey taking over airport security. That is the reality the West will face.

Telegraph Media Group Limited [2021]