Fears have been raised that ISIS will carry out a suicide attack at Kabul airport amid the chaos of the evacuation.
Ministers have admitted extremists could try to 'take advantage' of the desperate airlift to attack UK and US forces.
There are concerns that troops are 'having to keep their fingers on the trigger in one hand while holding a baby in the other' with desperate people trying to get away from the Taliban.
But Joe says it's safe to go to the airport...— Kayleigh McEnany (@kayleighmcenany) August 21, 2021
"ISIS terror threat forces US military to establish alternative routes to Kabul airport" ⬇️https://t.co/SHOuQMVv82
Defence minister James Heappey said suicide bombers were 'absolutely' one of the threats the operation faces.
'Absolutely and that is why - we would love to be able to just open the gates and let people in and even faster flow,' he told BBC Radio 4's Today programme.
'But there are people right now in Kabul trying to get onto British flights that we have identified in our checks as being on the UK no fly list.
'So the checks that are being done are entirely necessary because there are people trying to take advantage of this process to get into the UK to cause us harm.
'Secondly, there is a real threat that our troops are facing and we have to therefore execute this as a military mission, at the same time as being as humane as possible.'
A government source told The Times: '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. The soldiers are having to keep their fingers on the trigger in one hand while holding a baby in the other. It's very fragile.'
Military planes making evacuation runs into Kabul have been dropping flares and carrying out nosedive combat landings amid fears terrorists may try to shoot one down.
US military planes are doing rapid diving combat landings to beat the threat of a missile attack, with video showing a French transport plane yesterday deploying flares designed to confuse heat-seeking technology which may have been stolen by Islamic State in Afghanistan.
Taliban forces controlling all access points to Hamid Karzai International Airport are not thought to be attempting to shoot down military aircraft during the Western evacuation effort, as such an action could trigger another American-led intervention in Afghanistan.
It is feared Islamic State in Afghanistan - also known as ISIS-K - could use stolen heat-seeking missiles to bring down a rescue plane carrying hundreds of refugees including women and children.
Fears ISIS will mount suicide bombing at Kabul airport https://t.co/1MfFxmnfo3— Daily Mail Online (@MailOnline) August 23, 2021
ISIS militants have been fighting the Taliban for the last six years as they attempt to annex their own piece of Afghanistan following the collapse of their caliphate in Syria and Iraq following Western airstrikes and raids targeting the terror group.
Some 20 people are believed to have killed in the turmoil around Kabul airport over the past seven days.
© Associated Newspapers Ltd.