A U.S. drone strike killed a suicide car bomber who Pentagon officials said was preparing to strike Kabul airport on Sunday, as American forces worked to complete a withdrawal that will end two decades of military involvement in Afghanistan.

The strike was the second by the U.S. military since an Islamic State suicide bomb outside the airport on Thursday killed 13 U.S. troops and scores of Afghan civilians desperate to flee the country's new Taliban rulers.

The airport has been the scene of a massive airlift by U.S. and allied forces evacuating their citizens and at-risk Afghans that is due to wind up ahead of a Tuesday deadline set by U.S. President Joe Biden.

Officials said the strike targeted suspected militants from ISIS-K, a local affiliate of Islamic State that is an enemy of both the West and the Taliban movement that seized power on Aug. 15 after a lightning offensive.

One U.S. official said it was carried out by an unmanned aircraft and that secondary explosions showed the target had been carrying a substantial amount of explosives. Television footage showed black smoke rising into the sky.

U.S. officials had said they were particularly concerned about ISIS-K attacking the airport as American troops depart, in particular the threat from rockets and vehicle-borne explosives.

Biden said on Saturday that his military chiefs had told him another militant attack was highly likely.

The drone strike took place while remaining civilians waited at Hamid Karzai International Airport to be flown out before the last troops leave, a Western security official said. A U.S. official told Reuters on Saturday that fewer than 4,000 U.S. troops remained.

The Taliban said they had started their own investigations into the U.S. strike and whether the target was really a suicide bomber driving a vehicle loaded with explosives.

The United States and allies have taken about 114,400 people - including foreign nationals and vulnerable Afghans - out of the country in the past two weeks, but tens of thousands who want to go will be left behind.

"We tried every option because our lives are in danger. They (the Americans or foreign powers) must show us a way to be saved. We should leave Afghanistan or they should provide a safe place for us," said one woman outside the airport.

The airlift - one of the biggest such evacuation operations ever - marked the end of a 20-year Western mission in Afghanistan that began when U.S.-led forces ousted a Taliban government that had provided safe haven for the perpetrators of the Sept. 11, 2001, attacks on the United States.

The final chapter came after the United States and the Taliban made a deal last year to withdraw foreign troops. The Western-backed government and Afghan army then melted away as Taliban fighters swept across the country earlier this month.

A Taliban official told Reuters the Islamist group had engineers and technicians ready to take charge of the airport.

"We are waiting for the final nod from the Americans to secure full control over Kabul airport as both sides aim for a swift handover," said the official, speaking on condition of anonymity.

HONOURING THE DEAD

At a ceremony on Sunday at Dover Air Force Base in Delaware to honor members of the U.S. military killed in Thursday's attack, Biden shut his eyes and tilted his head back as the flag-draped transfer cases carrying the remains emerged from a military plane.

Crying could be heard and one woman collapsed. None of the fallen service members was over the age of 31, and five were just 20, as old as the war in Afghanistan itself.

Biden has vowed to go after the perpetrators and the United States said on Saturday it had killed two ISIS-K militants in a drone strike the day before. The Taliban condemned that strike, which took place in eastern Nangarhar Province bordering Pakistan, saying they should have been told about it in advance.

Biden's national security adviser, Jake Sullivan, told CBS' "Face the Nation" in an interview that Washington expected the Taliban to continue to allow safe passage for Americans and others to leave the country after the U.S. military withdrawal is completed.

The Afghan government's collapse leaves an administrative vacuum that has led to fears of an economic crisis and widespread hunger.

Prices for commodities including flour, oil and rice are rapidly rising and the currency is plunging, with money changers in Pakistan already refusing to accept the afghani.

On Saturday, officials ordered banks to reopen and imposed a limit on withdrawals of $200 or 20,000 afghani. Long lines forming outside bank branches.

Taliban spokesman Zabihullah Mujahid has said the group will announce a full Cabinet in the coming days, and that the difficulties will subside quickly once the new administration is up and running.

But with its economy shattered by decades of war, Afghanistan is now facing the end of billions of dollars in foreign aid poured in by Western donors.

The Taliban also appealed to the United States and other Western nations to maintain diplomatic relations after withdrawing. Britain said that should happen only if the Taliban allow safe passage for those who want to leave and respect human rights.

The Taliban's 1996-2001 rule was marked by a harsh version of sharia, Islamic law, with many political rights and basic freedoms curtailed and women severely oppressed.

Afghanistan was also a hub for anti-Western militants, and Washington, London and others fear it might become so again.

Biden has faced criticism at home and abroad for the chaos surrounding the final weeks of U.S. military presence in Afghanistan. He has defended his decisions, saying the United States long ago achieved its rationale for invading in 2001.