(Reuters) -- The U.S. and allies urged people to move away from Kabul airport on Thursday due to the threat of a terrorist attack by Islamic State militants as Western troops hurry to evacuate as many people as possible before an Aug. 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 U.S. and allied troops due to leave the airport next week.
In an alert issued on Wednesday evening, the U.S. embassy in Kabul advised citizens to avoid traveling to the airport and said those already at the gates should leave immediately, citing unspecified "security threats."
In a similar advisory, Britain told people in the airport area to move away and its armed forces minister, James Heappey, said intelligence of a possible suicide bomb attack by ISIS militants had become "much firmer."
"I can't stress the desperation of the situation enough. The threat is credible, it is imminent, it is lethal. We wouldn't be saying this if we weren't genuinely concerned about offering Islamic State a target that is just unimaginable," Heappey told BBC radio.
Australia also urged its citizens and visa holders to leave the area, warning of a "very high threat" of an attack.
A Western diplomat in Kabul said areas outside the airport gates continued to be "incredibly crowded" despite the warnings.
The Taliban, whose fighters are guarding the perimeter outside the airport, are enemies of the Afghan affiliate of Islamic State, known as Islamic State Khorasan (ISIS-K), after an old name for the region.
"Our guards are also risking their lives at Kabul airport, they face a threat too from the Islamic State group," said a Taliban official, who spoke on condition of anonymity.
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 Aug. 15.
While Western troops in the airport worked feverishly to move the evacuation as fast as possible, thousands of people still thronged outside, trying to flee rather than stay in a Taliban-ruled Afghanistan.
Ahmedullah Rafiqzai, a civil aviation official at the airport, said people continued to crowd around the gates despite the attack warnings.
"People don't want to move, it's their determination to leave this country that they are not scared to even die," he told Reuters.
A NATO country diplomat said that although the Taliban were responsible for security outside the airport, threats from Islamic State could not be ignored.
"Western forces, under no circumstances, want to be in a position to launch an offensive or a defensive attack against anyone," the diplomat added.
The other Western diplomat said flight operations had slowed on Wednesday but evacuations would be faster on Thursday.
It was unclear how many eligible people hoping to travel were left but one Western official said an estimated 1,500 U.S. passport and visa holders were trying to get to the airport.
The White House said President Joe Biden was briefed on Wednesday about the threat from the ISIS-K 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 a withdrawal 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 U.S. and its allies have mounted one of the biggest air evacuations in history, bringing out more than 88,000 people, including 19,000 on Tuesday. The U.S. military says planes are taking off the equivalent of every 39 minutes.
U.S. Secretary of State Antony Blinken said at least 4,500 American citizens and their families had been evacuated from Afghanistan since mid-August.
The U.S. military said it would shift its focus to evacuating its troops in the final two days before the Aug. 31 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 Taliban's 1996-2001 rule was marked by public executions and the curtailment of basic freedoms. Women were barred from school or work.
The Taliban have said they will respect human rights and not allow terrorists to operate from the country.
But, with the twentieth anniversary of the Sept. 11, 2001, attacks looming, Taliban spokesman Zabihullah Mujahid told NBC News there was "no proof" that al Qaeda's late leader Osama bin Laden was responsible.
U.S.-backed forces ousted the Taliban in late 2001 after their leadership refused to cave in to U.S. demands to make bin Laden leave Afghanistan.
"There is no evidence even after 20 years of war... There was no justification for this war," Mujahid said.