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KABUL: The US conducted a drone strike against an Islamic State target in Afghanistan on Saturday, as the airlift of those desperate to flee moved into a fraught final stage with fresh terror attack warnings and encroaching Taliban forces primed to take over Kabul airport.

US forces overseeing the evacuation have been forced into closer security cooperation with the Taliban to prevent any repeat of a suicide bombing that killed scores of civilians crowded around one of the airport’s main access gates and 13 American troops.

The attack was claimed by a regional chapter of Islamic State, and the Pentagon announced it had carried out a drone attack on a “planner” from the jihadist group in eastern Afghanistan.

“Initial indications are that we killed the target,” said Captain Bill Urban of the Central Command.

With the airlift window narrowing sharply ahead of an August 31 deadline, more than 5,000 people remain inside Kabul airport awaiting evacuation, and thousands more continue to throng the perimeter gates pleading for entry.

The carnage of Thursday’s suicide attack only injected further stress and tension into a situation already fraught with panic and despair for those wanting to leave and high risk for the US forces tasked with securing the operation.

The attack followed a chorus of warnings about an imminent threat and, as crowds gathered outside the airport Saturday, the United States issued a fresh alert for US citizens to leave the areas around the main gates “immediately”.

The warning came just hours after the Pentagon said the evacuation operation continued to face “specific, credible” threats.

“We certainly are prepared and would expect future attempts, absolutely,” Pentagon spokesman John Kirby told reporters.

The Pentagon clarified that there was one explosion Thursday, not two as previously believed.

At the White House, President Joe Biden’s press secretary Jen Psaki said US national security experts consider another attack is “likely” and the next few days will be “the most dangerous period to date”.

Taliban spokesman Bilal Karimi said on Twitter that fighters had moved into parts of the military side of Kabul airport, but the Pentagon pushed back, with Kirby saying gates and operations were still being run by the US military.

In one of the greater ironies after two decades of war, racing to meet the August 31 deadline for the US withdrawal has meant close cooperation with the Taliban on evacuee movements and the IS threat.

The head of US forces at the airport, Rear Admiral Peter Vasely, is in constant contact with the Taliban official overseeing security around the airport.

In most cases, US officials say, the Taliban has expedited the passage into the airport of foreign nationals, Afghans with visas to the United States and even Afghans who face threats from the Taliban due to their political or social activism or work for the media.

Under enormous criticism at home and abroad for his handling of the Afghan crisis and the US military withdrawal, Biden has pledged to stick to the airlift deadline and to punish those responsible for the suicide blast.

About 109,000 people have been flown out of the country since August 14, the day before the Taliban swept to power, according to the US government.

Some Western allies, including Britain and Spain, announced an end to their airlifts on Friday, following other nations such as Canada and Australia earlier in the week.

The United Nations said Friday it was bracing for a “worst-case scenario” of up to half a million more refugees from Afghanistan by the end of 2021.

The Taliban have promised a softer brand of rule compared with their first stint in power from 1996 to 2001, which ended when the United States invaded Afghanistan following the September 11 attacks.

But many Afghans fear a repeat of their brutal interpretation of Islamic law, as well as violent retribution for working with foreign militaries, Western missions or the previous US-backed government.

The role that women will be allowed to play in society has been one of the biggest concerns since the Taliban takeover, after women were banned from work and education and confined to the house during the group’s previous rule.

Taliban official Sher Mohammad Abbas Stanikzai, the former deputy chief negotiator of peace talks in Doha, said Friday that women have “an innate right” to work.

“They can work, they can study, they can take part in politics and they can do business,” he told a press conference.

The Taliban have allowed US-led forces to conduct the airlift while they finalise plans for their government to be announced as soon as the American troops have left.

But the Islamic State jihadists, bloody rivals of the Taliban with a record of barbaric attacks, were intent on capitalising on the chaos in Kabul.

In recent years, the Islamic State’s Afghanistan-Pakistan chapter has been responsible for some of the deadliest attacks in those countries.

It has massacred civilians at mosques, shrines, public squares and even hospitals.

“These are people that are even more extreme than the Taliban and are basically at war with the Taliban,” Australian Defence Minister Peter Dutton said.