LONDON (REUTERS, AFP, NYTIMES) - 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 (Aug 26).

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.

Mr Heappey confirmed that intelligence of a possible suicide bomb attack by Islamic State in Iraq and Syria (ISIS) 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," Mr Heappey told BBC radio.

"I think there is an appetite among 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 Taleban 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," Mr Heappey 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."

Earlier, London had issued a warning, saying that "if you can leave Afghanistan safely by other means, you should do so immediately".

The US had warned crowds trying to access Kabul airport to leave the area, with Australia also citing the "high threat" of a terrorist attack.

A flurry of near-identical travel warnings from London, Canberra and Washington late on Wednesday urged people gathered in the area to vacate and move to a safe location.

For days, thousands of fearful Afghans and foreigners have surrounded Kabul Hamid Karzai International Airport, in the hope of fleeing Taleban rule.

The security warnings about the airport were unusually specific.

"Those at the Abbey Gate, East Gate, or North Gate now should leave immediately," said the US State Department, citing unspecified security threats late on Wednesday.

The warnings came as the last of the estimated 1,500 Americans still in Afghanistan try to make it to the airport to leave before the US withdrawal on Aug 31. Tens of thousands of Afghan nationals are camped outside the perimeter of the airport in desperate attempts to escape on the last flights out.

Australia's department of foreign affairs said there was an "ongoing and very high threat of terrorist attack".

It said: "Do not travel to Kabul Hamid Karzai International Airport. If you're in the area of the airport, move to a safe location and await further advice."

It was not immediately clear what prompted the advisories, or whether they described a specific new threat or reflected ongoing concerns.

But a senior US official, who spoke on the condition of anonymity about confidential assessments, said the US was tracking a specific and credible threat at the airport from the Afghan affiliate of the ISIS, which has carried out dozens of attacks in recent years, many targeting ethnic minorities and other civilians.

The US government has been warning about potential security threats at the airport, and access to the airport has been adjusted accordingly, with some gates temporarily closed.

In its travel advisory currently at Level 4 (do not travel) - the highest level - US citizens still in Afghanistan have been advised to review their personal security plans, be aware of their surroundings and local security developments as well as keep a low profile.

On Thursday, huge crowds continued to throng the gates of Kabul airport despite the warnings of possible attacks by ISIS militants, a Western diplomat at the airport said.

The diplomat, who spoke on the condition of anonymity, said an estimated 1,500 US passport or visa holders were still trying to enter the airport. He said evacuation flights will pick up on Thursday after slowing down on Wednesday.

Washington and its allies have been flying thousands of Afghans out of the airport every day on hulking military transports, but it has become an increasingly difficult and desperate task.

Australia has been evacuating its citizens and visa holders for more than a week from Kabul airport, where Canberra had urged people to travel in order to be ready for transport.

Late on Wednesday, Australia changed its advice to those in the area, which Foreign Affairs Minister Marise Payne said was based on heightened concerns of an attack. "There is an ongoing and very high threat of a terrorist attack," Ms Payne told reporters in Canberra on Thursday.

The warning heightens the risk that scores of Afghans holding visas for Australia could be left behind as Canberra readies to end its evacuation programme.

Prime Minister Scott Morrison, who has previously said Australia will unlikely be able to evacuate everyone, declined to comment on whether Australia would continue flights up to the Aug 31 deadline the Taleban insists must be adhered to.

Mr Morrison said Australia has now evacuated about 4,000 people out of Afghanistan after another 1,200 people were flown out overnight. Many of these remain in the United Arab Emirates, he said, while 639 have been evacuated to Australia.

Australia was part of a Nato-led international force that battled the Taleban and trained Afghan security forces in the years after the militants were ousted in 2001. More than 39,000 Australian military personnel served in Afghanistan and 41 were killed there.

In Kabul, some evacuation flights are already winding down and are slated to end on Aug 31, leaving many clamouring for ways to leave the country. Crowds, including distraught families, have tried to access the airport which is ringed by Taleban and Western military checkpoints.

At least eight people have died in the airport chaos.

Many Afghans fear the Taleban will reprise their brutal brand of Islamist rule, which ended in 2001.

Washington said the Taleban had made assurances that Americans, "at-risk" Afghans and people from other nations would be allowed to leave even after the Aug 31 deadline for US troops to depart.

"They have a responsibility to hold to that commitment and provide safe passage for anyone who wishes to leave the country," Secretary of State Antony Blinken told reporters.

But Afghanistan is awash with other extremist groups - including the ISIS.

On Thursday, a Nato country diplomat in Kabul said that Taleban cadres have promised to provide security outside the airport, but intelligence reports of an imminent threat from ISIS militants cannot be ignored.

The diplomat, speaking on condition of anonymity, was referring to warnings by the US and Australia to its citizens to vacate the area outside the airport immediately because of the intelligence reports.

"Western forces, under no circumstances, want to be in a position to launch an offensive or a defensive attack against anyone in Afghanistan," the diplomat added. "Our mandate is to ensure evacuations end on Aug 31."

An official from Taleban said its guards continue to protect civilians outside Kabul airport, adding that Western forces must stick to a deadline of completing evacuations from Afghanistan by the end of the month.

"Our guards are also risking their lives at Kabul airport, they face a threat, too, from the Islamic State group," said the official, who spoke on condition of anonymity.