"Kabul is not right now in an imminent threat environment," Pentagon spokesman John Kirby told reporters, while acknowledging that Taliban fighters are "trying to isolate Kabul."

The Taliban on Thursday overran Afghanistan's second and third largest cities, Kandahar and Herat, days after the United States completed most of its withdrawal from a 20-year military involvement.

The Pentagon acknowledged its concerns but made clear that the United States believed that the Afghan military was now responsible.

"We have noted with great concern the speed with which they have been moving and the lack of resistance that they have faced, and we have been nothing but honest about that," Kirby said.

"We want to see the will and the political leadership -- the military leadership -- that's required in the field," he said.

"Whether it pans out or not, that's really for the Afghans to decide," he added. "No outcome has to be inevitable."