The general said his 'working assumption' is that a suicide bomber approached US service personnel near the Abbey Gate of the airport, where there was then an explosion followed by gunfire directed at US military personnel and civilians.

He said another bomb went off in the vicinity of the Baron Hotel, where the UK has been processing Britons and Afghans eligible for evacuation after the Taliban seized control of the nation.

Gen McKenzie said: 'The attack on the Abbey Gate was followed by a number of Isis gunmen who opened fire on civilians and military forces.'

'The threat from Isis is extremely real,' he said, adding the US will continue its evacuation mission and that he does not expect any further US troops will be flown in to assist.

An official in Kabul said at least 60 Afghans were killed and 143 others wounded in the airport attack.

Taliban spokesman Zabihullah Mujahid said his group 'strongly condemns' the attack and alleged it happened in an area under US control.

Transport Secretary Grant Shapps issued an aviation notice further advising airlines to avoid Afghan air space under 25,000 feet.

Armed forces minister James Heappey had earlier warned that there was 'very credible reporting' of an 'imminent' and 'severe' terror threat.

He had urged people queuing outside the airport to move to safety amid concerns surrounding the IS terror cell.

Alicia Kearns, a member of the foreign affairs and national security strategy committees, said there had been 'many hurt' in an attack near the Baron Hotel.

The Conservative MP tweeted: 'A bomb or attack with gun fire at northern gate of Baron's Hotel. Worried this will devastate evacuation - so many hurt. My heart is with all those injured and killed.'

A local witness told PA he heard the explosion by Kabul airport as he was walking to evening prayer.

Ahmad, whose name has been changed for security reasons, said he is safe as he was some distance from the blast.

'First was explosion, and then firing started, I mean... heavy gunfire,' he said. 'But even far away people were running... there's alarm.'

Labour leader Sir Keir Starmer said the reports from Kabul were 'devastating', adding: 'Our thoughts are with all those killed and wounded, serving personnel supporting the evacuations and all those desperately trying to leave.'

The evacuation effort was already under extra strain after a warning on Wednesday night from the Foreign Office for UK nationals near the airport to leave and head for safety due to the 'ongoing and high threat of terrorist attack'.

Mr Johnson had vowed 'we'll do everything we can to get everybody else' before the deadline for British troops to depart in advance of the exit of US forces, after President Joe Biden refused his request to extend the time frame.

But the PM conceded that although the 'lion's share' of eligible Afghans have been removed from the country, 'there will be people who still need help'.

The US is providing security at Kabul airport, meaning other allied forces are expected to have to wind down their evacuation efforts and depart ahead of the Americans.

Mr Heappey had said eight RAF flights managed to lift 1,988 people from Kabul within the past 24 hours, taking the total since the Taliban began its march to power to 12,279.

Pontins has been approach for comment.