U.S. troops have begun their withdrawal from Kabul airport, the Pentagon said on Saturday, as the evacuation efforts from the Afghan capital entered the 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 U.S. troops securing Hamid Karzai International Airport, where an unprecedented airlift operation is set to end by Tuesday.

After a U.S. 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.

U.S. officials have said that as troops are flown out there is increasing concern about the threat posed to the airport by militants from the Islamic State-Khorasan (ISIS-K) group, with a focus on rocket attacks and vehicle-borne explosives.

Following an Islamic State suicide bombing on Thursday that killed scores of Afghan civilians and 13 U.S. troops, the U.S. military launched a drone strike on Friday that it said targeted members of the group in Nangarhar Province.

U.S. 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, Reuters reported.

The White House said 2,000 people were taken out of Kabul between 12 noon and midnight on Saturday, appearing to have slowed from the 6,800 people evacuated in the previous 24 hours, Bloomberg reported.

According to the report the White House Deputy Press Secretary Karine Jean-Pierre said a total of 113,500 were airlifted out of Kabul since Aug. 14.