Boris Johnson vowed to continue the evacuation effort in Afghanistan as the Pentagon confirmed 11 US marines and a navy medic were killed in attacks on Thursday described by the UK Prime Minister as “barbaric”.
A message posted on the Islamic State’s (IS) news outlet's Telegram channel said a suicide bomber, whom they identified, had detonated an explosives vest among Afghans and US forces.
The MoD had earlier issued warnings about the so-called IS splinter cell, Isis-K, which is opposed by the US and the Taliban.
The first explosion took place nearby to the closed Abbey Gate outside Hamid Karzai International Airport.
The second blast was close to the Baron Hotel, where British military personnel and journalists had been staying.
In a tweet, the Ministry of Defence (MoD) said there had been no reported UK casualties, military or civilian.
Processing facilities at the Baron Hotel have now been closed.
Dozens of casualties were being treated at a surgical centre close to the airport, according to Emergency, a charity that runs a network of war hospitals and first aid posts across Afghanistan.
Pentagon spokesman John Kirby confirmed the explosions were part of a “complex attack”.
“We can confirm that a number of US service members were killed in today’s complex attack at Kabul airport,” he said.
"A number of others are being treated for wounds. We also know that a number of Afghans fell victim to this heinous attack.
“Our thoughts and prayers go out to the loved ones and teammates of all those killed and injured.”
After chairing an emergency Cobra meeting on Thursday afternoon, Mr Johnson
insisted the UK’s evacuation effort in Afghanistan would continue despite the attack, which he labelled “barbaric”.
"The conclusion is that we’re able to continue with the programme in the way that we’ve been running it, according to the timetable we’ve got,” he said.
“That’s what we’re going to do because the overwhelming majority of those who are eligible have now been extracted from Afghanistan.
“We’re going to work flat out, the military, the Foreign Office teams, the Home Office, the Border Force teams that are there getting people through as fast as they can still.
“We’re going to keep going up until the last moment, but I want to repeat what I’ve been saying over the last few days. We also fully expect that those who want to leave Afghanistan after this phase one, if you like, are allowed to do that by the Taliban.
“We will use all the influence that we can bring to bear with the political or economic, diplomatic, as we said at the G7, to encourage the new authorities in Afghanistan to do that.”
First Minister Nicola Sturgeon offered her condolences to the victims of the bombing attacks.
Posting on her Twitter account on Thursday evening, Ms Sturgeon wrote: “The dreadful attacks at Kabul airport today compound the horror unfolding in Afghanistan.
“My thoughts are with those bereaved and injured.”
She added: “We owe such a debt to our armed forces and others working against the clock to evacuate as many people as possible from this grim situation.”
The United States and its allies, including the UK, had been racing to airlift their citizens and some Afghans from Kabul before its military is set to fully withdraw from Afghanistan on Tuesday.
The explosions come after several countries involved in the evacuations urged people to avoid the airport.
In a tweet, the MoD said: “There have been no reported UK military or UK Government casualties following the incidents in Kabul.
“UK forces are working closely with our partners to provide security and medical assistance.
“Our primary concern remains the safety of our personnel, British citizens and the citizens of Afghanistan.”
It added: "We are in close contact with our US and other Nato allies at an operational level on the immediate response to this incident.”
Marine Corps General Kenneth McKenzie, commander of US Central Command, said America was continuing its evacuation mission despite the attacks.
About 1,000 US citizens are still believed to be in Afghanistan.
Asked how the bombers were able to approach US forces, Gen McKenzie said there had "clearly been a failure” from the Taliban forces checking people outside the airport.
Taliban spokesman Zabihullah Mujahid said his group “strongly condemns” the attack and alleged it happened in an area under US control.
UK foreign secretary Dominic Raab offered condolences to the US over the deaths of 12 service members, saying they had “paid the ultimate sacrifice”.
He said: “Today’s attack is a stark reminder of the dangerous situation in which UK military and civilian personnel have been working so hard to evacuate people and we should be proud of their bravery and what they have achieved.
“The UK and US remain resolute in our mission to get as many people out as possible. It is testament to the remarkable courage of our personnel that they continue to do so while under fire. We will not let the cowardly acts of terrorists stop us.”
UK 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 there was “very credible reporting” of an “imminent” and “severe” terror threat.
He had urged people queuing outside the airport trying to flee the Taliban to move to safety amid concerns surrounding the IS terror cell, following confirmation Britain had 11 flights scheduled out of Kabul on Thursday.
Labour leader Sir Keir Starmer described the Kabul airport attack as “devastating”.
Writing on Twitter, he said: “Devastating reports from Kabul. Our thoughts are with all those killed and wounded, serving personnel supporting the evacuations and all those desperately trying to leave.
“The bravery of the troops at the airport will always outweigh the cowardice of those who wish to harm us.”
Conservative MP Nus Ghani said she was on the phone to somebody outside Kabul airport when the first explosion happened.
The Wealden MP tweeted: “Explosion at Kabul airport. I was on the phone to an Afghan outside the airport when he heard the explosion.
“Praying that he gets away safely and we get his family safe passage out of this nightmare.”
Conservative MP Alicia Kearns, a member of the foreign affairs and national security strategy committees, said there had been “so many hurt” in a bomb or gun attack at the Baron Hotel in Kabul.
She 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.”
She added: “No I’m not just worried, I’m livid, and I’m heartbroken and I’m furious. The deals being done without UK input by ‘allies’. Taliban checkpoints stopping our people, but not terrorists. There are people – damn good people – who for days have been trying to get out, now fleeing.”
French president Emmanuel Macron said he would co-ordinate with “our American allies” in response to the explosions.
Speaking in French on a bilateral visit to Dublin, he said: “As we speak in front of you, the situation is worsening around the the military airport.
“We are being confronted with a very tense situation, which leads us to co-ordinate with our American allies.
“We will also closely co-ordinate on the issues to be dealt with in the near future, military co-operation, migration issues and co-operate with the UN Security Council, because in the coming days and weeks we will have to define the course of the mandate of the United Nations.
“The coming hours will remain extremely dangerous in Kabul and around the airport.”
On Friday morning, Defence Secretary Ben Wallace has said the UK evacuation mission in Kabul is into its final "hours" after closing the main processing centre in Baron Hotel near the airport.
He told Sky News: "We at 4.30 this morning, UK-time, closed the Baron's hotel, shut the processing centre and the gates were closed at Abbey Gate.
"We will process the people that we've brought with us, the 1,000 people approximately in the airfield now and we will seek a way to continue to find a few people in the crowds where we can, but overall the main processing is now closed and we have a matter of hours."
He added: "The sad fact is not every single one will get out."
Mr Wallace also insisted that the Kabul airport terror attack "didn't hasten our departure" and the main UK evacuee processing site was closed "almost exactly on schedule".
"The threat is obviously going to grow the closer we get to leaving," he told Sky News.
"The narrative is always going to be certain groups, such as IS, will want to stake a claim that they have driven out the US or the UK.
"We closed the Baron's hotel almost exactly on schedule. The explosion was horrendous, but it didn't hasten our departure."
Nearly 14,000 British nationals and Afghans were rescued in the mission since the middle of August, Mr Wallace said.
This comes after the Foreign Office defended its Afghanistan embassy staff after documents identifying Afghan workers and job applicants were found left on the ground at the British diplomatic mission in Kabul.