Spokespeople at the embassy in Kabul and at the State Department in Washington did not respond to messages Sunday.
The security alert said Americans who wanted assistance leaving Afghanistan would have to register through a repatriation form and should not try to contact the embassy.
Hamid Karzai International Airport, on the north side of the city, is just miles from the U.S. Embassy. Helicopters ferry passengers to the airport in a matter of minutes.