The United States and allies urged people to move away from Kabul airport on Thursday due to the threat of an Islamic State terror attack as Western troops hurry to evacuate as many Afghans as possible before an August 31 deadline.

Pressure to complete the evacuations of thousands of foreigners and Afghans who helped Western countries during the 20-year war against the Taliban has intensified with all US and allied troops due to leave the airport next week.

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

Britain issued a similar advisory, telling people in the area of the airport to "move away to a safe location".

"There is an ongoing and high threat of terrorist attack", the British Foreign Office statement said.

Australia also urged its citizens and those with a visa for Australia to leave the area, warning of a "very high threat of a terrorist attack" at the airport.

The warnings came against a chaotic backdrop in the capital, Kabul, and its airport, where a massive airlift of foreign nationals and their families as well as some Afghans has been underway since the Taliban captured the city on August 15.

While Western troops inside the airport worked feverishly to keep the evacuation moving as fast as possible, Taliban fighters guarded the perimeter outside where thousands of people have thronged, trying to flee the country rather than stay in a Taliban-ruled Afghanistan.

"It's very easy for a suicide bomber to attack the corridors filled with people and warnings have been issued repeatedly," Ahmedullah Rafiqzai, an Afghan official working at the Directorate of Civil Aviation at the Kabul airport, told Reuters.

"But people don't want to move, its their determination to leave this country that they are not scared to even die, everyone is risking their life."

Taliban fighters have promised to provide security outside the airport, but intelligence reports of an imminent threat from Islamic State militants cannot be ignored, a NATO country diplomat in the Afghan capital said on Thursday.

The White House said President Joe Biden was briefed on Wednesday about the threat from the ISIS-K militant group as well as contingency plans for the evacuation.

Biden has ordered all troops out of Afghanistan by the end of the month, to comply with an agreement with the Taliban, despite European allies saying they needed more time to get people out.

In the 11 days since the Taliban swept into Kabul, the United States and its allies have mounted one of the biggest air evacuations in history, bringing out more than 88,000 people, including 19,000 in the past 24 hours. The US military says planes are taking off the equivalent of every 39 minutes.

US Secretary of State Antony Blinken said at least 4,500 American citizens and their families had been evacuated from Afghanistan since mid-August, and the State Department was reaching out to about 1,500 who remained there.

Blinken told a news conference in Washington there was no deadline for the effort to help people who want to leave, both Americans and others, and that it would continue for "as long as it takes."

The US military said it would shift its focus to evacuating its own troops in the final two days before the August 31 withdrawal deadline.

The Taliban have said foreign troops must be out by the end of the month. They have encouraged Afghans to stay while saying those with permission to leave will still be allowed to do so once commercial flights resume after the foreign troops go.

The militant group has asked NATO member Turkey to help keep the airport open after foreign troops leave. Turkey said technical experts might remain to help operate the airport.

The United Nations is leaving some 3,000 Afghan staff at its mission. A UN security document reviewed by Reuters described dozens of incidents of threats, the looting of UN offices, and physical abuse of staff since August 10.

The Taliban's 1996-2001 rule was marked by public executions and the curtailment of basic freedoms. Women were barred from school or work.

The US-backed Afghan government collapsed swiftly after Biden withdrew the troops, two decades after US-back forces had ousted the Taliban in the weeks following September 11, 2001, attacks on the United States, planned from Afghan territory by al Qaeda.

While the Taliban have said they will respect human rights and not allow terrorists to operate from the country, Taliban spokesman Zabihullah Mujahid told NBC News there was "no proof" that Osama Bin Laden, the late Al Qaeda chief, was involved in the 9/11 terror attacks on New York and Washington.

"There is no evidence even after 20 years of war, we have no proof he was involved ... There was no justification for this war," he said.