An explosion has been heard outside Kabul airport following warnings that a terror attack could be launched in the final phases of the evacuation effort.

US defence spokesman John Kirby confirmed the blast occurred and said “casualties are unclear at this time” on Thursday afternoon.

At least 13 people including children have been killed, a Taliban official told Reuters.

The spokesperson added that many Taliban guards have also been injured in the explosion.

It came after armed forces minister James Heappey previously warned there is “very credible reporting” of an “imminent” and “severe” threat to Kabul airport.

Following the explosion close to Kabul airport, the Ministry of Defence tweeted: “We are working urgently to establish what has happened in Kabul and its impact on the ongoing evacuation effort.

“Our primary concern remains the safety of our personnel, British citizens and the citizens of Afghanistan. We are in close contact with our US and other Nato allies at an operational level on the immediate response to this incident."

Former British Royal Marine targeted

A former Royal Marine who was near to the explosion close to Kabul airport has said his vehicle was targeted by a gunman amid the chaos.

Paul Farthing, known as Pen, who founded the Nowzad shelter in Kabul, is aiming to get 200 dogs and cats out of the country alongside his animal shelter staff.

Mr Farthing, who was outside the airport in a car when the incident occurred, told the PA news agency: “We’re fine but everything is chaos here at the moment.

“All of a sudden we heard gunshots and our vehicle was targeted, had our driver not turned around he would have been shot in the head by a man with an AK-47.

“We’ve been in the airport, and back out of the airport; the whole thing’s a mess.

“There’s not much more I can say at the moment, I need to make sure the animals and everyone is safe.”

Kabul explosion ‘appalling and cowardly’

Shadow foreign secretary Lisa Nandy said the attack outside Kabul airport was “appalling and cowardly”.

She tweeted: “Devastating reports from Kabul. This is an appalling and cowardly attack on those already fleeing unimaginable horrors. My thoughts are with the Afghan people and the British, US and international personnel who have remained at the airport to save as many lives as possible.”

Prime Minister Boris Johnson has been updated on the situation at Kabul airport and will chair an emergency Cobra meeting on Thursday afternoon, Downing Street has said.