Facing an Aug. 31 deadline to leave Afghanistan, the U.S. must shift in a few days from evacuating thousands of Afghans and up to 1,500 Americans still in the country to the job of loading the remaining U.S. military personnel and essential equipment onto the final departing planes.

The evacuation operation in Kabul is taking place under increasingly perilous conditions. The U.S. Embassy in Kabul, citing terror threats, issued a security alert late Wednesday to tell Americans not to travel to Hamid Karzai International Airport and said those already there should leave.

“U.S. citizens who are at the Abbey Gate, East Gate, or North Gate now should leave immediately,” the alert said. Officials have been warning that members of the Afghan branch of the Islamic State extremist group were trying to mount an attack on military personnel or civilians at the airport.

The Biden administration on Wednesday provided its first count of American civilians in Afghanistan, saying as many as 1,500 U.S. citizens remained in the country.