US Pentagon spokesman John Kirby has confirmed there has been an explosion outside Kabul airport, adding that “casualties are unclear at this time”.

Mr Kirby took to Twitter to confirm reports of an explosion as thousands try to flee Afghanistan after the Taliban took control of the capital.

He said: “We can confirm an explosion outside Kabul airport. Casualties are unclear at this time. We will provide additional details when we can.”

Boris Johnson: “We’re doing everything we can”

It comes as Boris Johnson warned time is running out for the evacuation effort in Kabul as the operation was strained by a warning that a lethal terror attack could be launched within hours.

The Prime Minister said on Thursday morning that the “overwhelming majority” of eligible people have now been evacuated from Afghanistan, but he conceded the time left is now “quite short”.

He 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 on Tuesday, after President Joe Biden refused his request to extend the time frame.

Kabul airport safety concerns

Mr Johnson’s pledge came after armed forces minister James Heappey warned there is “very credible reporting” of an “imminent” and “severe” threat to Kabul airport.

Mr Heappey called on those queuing outside Hamid Karzai International Airport to move to safety amid concerns over an affiliate of the so-called Islamic State in Afghanistan, known as Isis-K.

He said Britain has 11 flights scheduled out of Kabul on Thursday but declined to say whether that will be the end of the operation, citing the security of troops on the ground.

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.

Boris Johnson on Isis-K

The Prime Minister told reporters: “In the time we have left, which may be – as I’m sure everybody can appreciate – quite short, we’ll do everything we can to get everybody else.”

He also warned of the threat being posed by Isis-K, while speaking on a visit to the Permanent Joint Headquarters in north London, where he met military personnel co-ordinating the evacuation effort.

“I think we have to be transparent about the risks, that we have to be realistic about what’s going on, and you’ll appreciate that there are Islamic State Khorasan province terrorists out there,” Mr Johnson said.

“I can’t go into the details, clearly. But we have to be mindful of the security of our personnel, but also of the Afghan people who are trying to get out.”