WASHINGTON (NYTIMES) - President Joe Biden's national security adviser warned on Sunday (Aug 22) that the threat of a terrorist attack by the Islamic State in Iraq and Syria (ISIS) posed a serious danger to the administration's evacuation of thousands of Americans and Afghan allies from the international airport in Kabul.

"The threat is real. It is acute. It is persistent. And it is something that we are focused on with every tool in our arsenal," Mr Jake Sullivan, the national security adviser, said on CNN's State of the Union.

His comments were the most urgent so far regarding a range of steadily increasing threats that intelligence and military officials have privately briefed Mr Biden and his top aides on in recent days, officials said.

Neither Mr Sullivan nor other senior American military or intelligence officials provided details about the threats or their specificity.

Current and former officials say, however, that they range from a missile attack against a transport plane taking off or landing at Hamid Karzai International Airport to a bomb-laden truck or suicide bombers infiltrating the crowd outside the airport.

Mr Biden said on Friday that US military and counter-terrorism officials were closely watching for threatened ISIS attacks, noting that thousands of prisoners had been released in Kabul and other locations.

While the Taleban is unlikely to have consciously let out the Islamic State in Iraq and Syria (ISIS) fighters, the chaos in Afghanistan in recent weeks allowed all manner of prisoners to be freed from custody, including the Taleban's enemies.

"ISIS-K has been waiting for an opportunity like this, where its fighters can exploit the chaos of the situation on the ground for a chance to kill American soldiers," said Dr Colin P. Clarke, a counter-terrorism analyst at the Soufan Group, a New York-based security consulting firm, referring to the Islamic State in Iraq and Syria's (ISIS) Khorasan affiliate in Afghanistan.

For the past week, US officials have warned about threats against the airport and American operations to evacuate civilians from Kabul. By last Friday, officials said, the threat reporting was growing more acute.

An attack on the airport, current and former officials said, would be a strategic blow against both the United States and the Taleban, which is trying to demonstrate that it can control the country.

The Taleban have fought ISIS in recent years, and leaders of ISIS in Afghanistan denounced the Taleban takeover of the country, criticising their version of Islamic rule as insufficiently hardline.

Mr Sullivan said that American commanders on the ground were using "a wide variety of capabilities" to defend the airfield against an attack, and were working closely with spy agencies to identify and defeat any threats. He did not provide details.

"It is something that we are placing paramount priority on stopping or disrupting," Mr Sullivan said. "And we will do everything that we can for as long as we are on the ground to keep that from happening. But we are taking it absolutely deadly seriously."

Indeed, American officials said last Saturday that the US military was establishing alternative routes to the Kabul airport for Americans, Afghan allies and citizens from other Western nations because of the threat ISIS posed to the airfield and its surroundings, a development previously reported by CNN.

Last Thursday night, CH-47 Chinook helicopters picked up 169 Americans at a hotel meeting place and whisked them to safety, rather than having them walk 182m to an airport gate where a large, unruly crowd had gathered.

American and Western cargo planes taking off from the airport have dispensed flares and chaff, a common precautionary practice in conflict zones like Afghanistan and Iraq to fool heat-seeking missiles fired from the ground, military officials said.

US military officials in Kabul who are communicating with senior Taleban leaders to provide safe passage to the airport for Americans and Afghan allies are also in this rare instance of cooperation to thwart a common enemy: ISIS.

A deadly attack against American and Afghan civilians would be a disaster not only for the United States, but also for the Taleban, which is moving to consolidate control over Kabul.

The Taleban and ISIS have been enemies, fighting each other on the battlefield for control of parts of the country.

Western counter-terrorism analysts say a high-profile attack by ISIS during the evacuation would most likely lift the group's flagging fortunes, recruiting and prestige.

A United Nations report in June assessed that ISIS' "territorial losses have affected the group's ability to recruit and generate new funding".

Although the ISIS affiliate was still believed to have 1,500 to 2,200 fighters in small areas of Kunar and Nangarhar provinces, the report said, "it has been forced to decentralise and consists primarily of cells and small groups across the country, acting in an autonomous manner while sharing the same ideology".

While the group suffered military setbacks starting in summer 2018, the report concluded that since June 2020, under its ambitious new leader, Shahab al-Muhajir, the affiliate "remains active and dangerous", and is seeking to swell its ranks with disaffected Taleban fighters and other militants.

"Given that ISIS-K and the Taleban are enemies, it will be a challenge for ISIS-K," Dr Clarke said. "Nevertheless, the Taleban now has its hands full with governing, which will consume considerable bandwidth within the organisation."

Mr Nathan Sales, the State Department's counter-terrorism coordinator in the Trump administration, said on Sunday that if the ISIS affiliate was capable of attacking the airport in Kabul, "that suggests Afghanistan after the US withdrawal will be a permissive environment for all sorts of terrorist groups, even those hostile to the Taleban".

Defence Department officials have been tight-lipped about the threats and what they are doing to defeat them, citing operational security.

Mr John Kirby, the chief Pentagon spokesman, declined on Saturday to comment on any specific threat information, but he acknowledged that the security situation at the airport was extremely volatile.

"I'm not going to get into specific threat assessments," Mr Kirby said. "The situation in Kabul, in the whole city, is fluid and dynamic. And it changes. It changes almost by the hour, and it changes in locations around the airport. It's very, very fluid and dynamic."