Fears are reportedly mounting that Isis militants will target US and UK soldiers restraining crowds at Kabul airport in Afghanistan as evacuation chaos continues.
Troops are now facing a "serious threat" of an Isis suicide bomber, a government source told The Times.
They said: “We know they would love to get a suicide bomb into the crowd and take out some Brits or Americans. There is a serious threat of an Isis suicide bomber."
It comes as 900 British soldiers have been sent to the country to help US forces manage their withdrawal.
Kabul airport has descended into chaos and panic since the Taliban swept across the country and seized power on 15 August, as people try to leave the country.
US president Joe Biden has been criticised for his decision to pull troops out by 31 August but has stood by the move.
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Footage from last week showed hundreds of people clinging to a US Air Force plane as it moved to take off and Afghans passing babies to US and UK soldiers in a bid to get them to safety.
Meanwhile, former British ambassador to the US Lord Kim Darroch said there “clearly” should have been more coordination and planning for an “orderly” exit from Kabul.
Darroch told Radio 4’s Today programme the soldiers “in these appalling circumstances are doing an extraordinary job”, adding that the situation is “extraordinarily fragile”, with Taliban forces and British and American forces “literally yards apart”.
He also said the UK has "rather passively acquiesced" in a foreign policy disaster.
Darroch said: “It’s going to take quite a long time for the West as a whole – because it’s a Western failure, a Western disaster, not just the UK and the US – to recover from all this, to recover our reputation."
Boris Johnson will push for sanctions against the Taliban at a G7 meeting on Tuesday.
Britain believes the G7 should consider economic sanctions and withhold aid if the Taliban commits human rights abuses and allows its territory to be used as a haven for militants, according to a British government official who spoke to Reuters on condition of anonymity.
He 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."
Elsewhere, foreign secretary Dominic Raab is still under fire for remaining at his luxury holiday in Crete as the crisis in Kabul unfolded.
A fresh wave of calls for Raab to resign has followed reports that a senior Downing Street official told him to return to London immediately on Friday 13 August as the Taliban advanced on Kabul.
According to reports in the Sunday Times and Mail on Sunday, Johnson assured him he could stay on holiday until Sunday, and he eventually returned in the early hours of Monday morning. However, a close ally of Raab's denied these reports.
Defence secretary Ben Wallace said the Kabul evacuation effort is “down to hours now, not weeks” as he conceded the UK’s involvement will end when the US leaves.
A Taliban spokesman has told Sky News that if the evacuation effort continues beyond the end of the month, it would be a “red line” and would “provoke a reaction”.
Suhail Shaheen said: “This is something … you can say it’s a red line. President Biden announced this agreement that on August 31 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.
“It will create mistrust between us. If they are intent on continuing the occupation, so it will provoke a reaction.”
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