US troops have begun their withdrawal from Kabul airport, the Pentagon said on Saturday, as the evacuation efforts from the Afghan capital entered their final stages. President Joe Biden sent thousands of troops to the airport as the Taliban swept through Afghanistan earlier this month to help evacuate American citizens, at-risk Afghans and other foreigners desperate to flee. At the peak of the deployment there were 5,800 US troops securing Hamid Karzai International Airport, where an unprecedented airlift operation is set to end by Tuesday. After a US official told Reuters there were fewer than 4,000 troops left at the airport, Pentagon spokesperson John Kirby confirmed to reporters at a briefing that the withdrawal had begun. He declined to say how many service members remained. US officials have said that as troops are flown out there is increasing concern about the threat posed to the airport by militants from the ISIS-Khorasan (ISIS-K) group, with a focus on rocket attacks and vehicle-borne explosives. Following an ISIS suicide bombing on Thursday that killed scores of Afghan civilians and 13 US troops, the US military launched a drone strike on Friday that it said targeted members of the group in Nangarhar Province, east of Kabul. US Army Major General William Taylor, from the military’s Joint Staff, told the briefing two “high-profile” ISIS-K planners and facilitators were killed and another wounded. The Pentagon declined to give further details. US officials told Reuters that neither of those killed were believed to be senior militants, and that it was likely there would be further military operations against the group in the coming days and weeks. Despite Biden’s vow to go after the perpetrators of Thursday’s attack, US officials have cautioned that beyond a symbolic act or limited operation, the United States could in fact do little to degrade ISIS-K. “We’ve been trying to destroy the group in Afghanistan since 2014 and couldn’t do it with thousands of troops on the ground,” a defense official said. As the evacuation operation nears its end, a State Department spokesman said on Saturday that about 350 Americans are still trying to leave the country. Some 5,400 US citizens have been evacuated since Aug. 14, according to the US government.