There is "very, very credible" intelligence that militants are planning an imminent attack on those gathering at Kabul airport in an attempt to flee Afghanistan, British armed forces minister James Heappey said on Thursday.

Late on Wednesday, Britain's foreign ministry advised people not to travel to the Kabul Hamid Karzai International Airport where thousands are waiting for flights out of the country ahead of an Aug. 31 deadline when the United States and its allies will pull out their remaining troops.

Heappey confirmed that intelligence of a possible suicide bomb attack by Islamic State militants had become "much firmer".

"There is now very, very credible reporting of an imminent attack, and hence why the Foreign Office advice was changed last night, that people should not come to Kabul Airport, they should move to a safe place and await further instructions," Heappey told BBC radio.

"I think there is an appetite amongst many in the queue to take their chances, but the reporting of this threat is very credible indeed. There is a real imminence to it."

He said that Western nations were relying on the Taliban for security outside the airport, and that despite the warnings there were a large number of people still waiting there.

"I can only say that the threat is severe. We will do our best to protect those who are there," he said. "There is every chance that as further reporting comes in, we may be able to change the advice and process people anew, but there is no guarantee of that."