The Royal Australian Air Force Air Load Team assist evacuees into their seats in Kabul, Afghanistan. Reuters

Western nations warned their citizens on Thursday to immediately leave the surrounds of Kabul airport over a terrorist threat as Taliban cadres have promised to provide security outside Kabul airport, but intelligence reports of an imminent threat from Daesh militants cannot be ignored.

In an alert issued on Wednesday evening, the US embassy in Kabul advised citizens to avoid travelling to the airport and said those already at the gates should leave immediately, citing unspecified "security threats".

Hundreds of people gather near an evacuation control checkpoint during ongoing evacuations in Kabul. AP

In a similar advisory, Britain told people in the airport area to move away.

"There is now very, very credible reporting of an imminent attack," British armed forces minister James Heappey told BBC radio.

"People should not come to Kabul airport, they should move to a safe place and await further instructions."

Meanshile, Taliban guards continue to protect civilians outside Kabul airport, spokesman, adding that Western forces must stick to a deadline of completing evacuations from Afghanistan by the end of the month.

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

Australian citizens and visa holders prepare to board the Royal Australian Air Force aircraft in Kabul. Reuters

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

Nearly 90,000 Afghans and foreigners have fled Afghanistan via the US-led airlift since the Taliban movement took control of the country on August 15; 19,000 in just 24 hours.

The United States warned crowds trying to access Kabul airport to leave the area, as Britain and Australia cited the "high threat" of a terrorist attack.

US Air Force loadmasters load passengers in support of the Afghanistan evacuation in Kabul. AFP

A flurry of near-identical travel warnings from London, Canberra and Washington late 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's Hamid Karzai International Airport, in the hope of fleeing Taliban 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."

A Taliban fighter patrolling in Kabul, Afghanistan, on Wednesday. AP

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

"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."

London issued a similar warning, adding "if you can leave Afghanistan safely by other means, you should do so immediately."

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.