Joint Chiefs Chairman Gen Mark Milley said a "significant" number of interpreters and other employees could be targeted by Taliban militants.
As many as 18,000 Afghan nationals have applied for US visas to immigrate to the US under a special programme.
But the scheme has been hit by delays, with applicants waiting for years.
"We recognise that there are a significant amount of Afghans that supported the United States and supported the coalition, and that they could be at risk," Gen Milley said in remarks his office released on Thursday.
