People familiar with the operation said very few individuals beyond U.S. citizens had been permitted to gain airport access over the weekend, as the Pentagon winds its mission down and the overall pace of relocations slows.

Instead, many Afghans who applied for or obtained visas through the SIV program were turned away or unable to get through the airport gates controlled by U.S. and other foreign military forces. Even some Afghans with green cards, which allow them to reside permanently in the United States, have been unsuccessful, these people said.

A U.S. official, who spoke on the condition of anonymity to discuss internal deliberations, said there had been no policy decision to exclusively admit American citizens.

But the official acknowledged that security threats had made it harder to facilitate entry for those who are not citizens, especially since a massive suicide bombing Thursday killed 13 service members assisting with the evacuation and about 170 mostly Afghan civilians.