Following an alert of “potential security threats” outside the gates of Kabul’s airport on Saturday, U.S. defense officials are warning that the Islamic State affiliate ISIS Khorasan may be trying to target American citizens near the evacuation center in Afghanistan.

“We are advising U.S. citizens to avoid traveling to the airport and to avoid airport gates at this time unless you receive individual instructions from a U.S. government representative to do so,” the U.S. Embassy in Afghanistan wrote Saturday.