Another attack on Kabul airport is highly likely, US President Joe Biden has warned, saying commanders have told him it could come as early as Sunday.
The state department has urged all US citizens to leave the area near the airport because of a “specific, credible threat”.
The US is continuing evacuations but the final UK troops, diplomats and officials have now left Kabul.
A suicide bombing near the airport on Thursday resulted in some 170 deaths.
A local branch of the Islamic State group – Islamic State in Khorasan Province (IS-K) – claimed the attack.
In retaliation, the US carried out a drone strike on eastern Afghanistan late on Friday, saying it had killed two “high-profile” IS-K members.
The two are described as a planner and a facilitator. It is unclear whether they were directly involved in planning the Kabul airport attack.
“This strike was not the last. We will continue to hunt down any person involved in that heinous attack and make them pay,” Mr Biden said in a statement released on Saturday.
IS-K is the most extreme and violent of all the jihadist militant groups in Afghanistan and has major differences with the Taliban, who now control most of the country. It accuses them of abandoning the battlefield in favour of a negotiated peace settlement with the Americans.
The Taliban condemned the air strike, saying the Americans should have consulted them first, a spokesman told Reuters news agency.
The next few days of the US evacuation operation are likely to be the most dangerous since it began, White House officials say.
US troops have begun their withdrawal from the airport – their numbers are now down to 4,000, from a peak of 5,800 in the past week.
One Western security official told Reuters that US forces were in the final phase, although a time for the end of the operation had yet to be decided.
Just over 1,000 civilians remain to be airlifted, the official said.
A Taliban official told Reuters their technical experts and engineers were ready to take over the airport when they were given the “final nod from the Americans”.