"Kabul is not right now in an imminent threat environment," Pentagon spokesman John Kirby told reporters.

"But clearly," he said, "if you just look at what the Taliban has been doing you can see that they are trying to isolate Kabul."

"It is not unlike the way they have operated in other places of the country, isolating provincial capitals and sometimes being able to force a surrender without necessarily much bloodshed," Kirby said.

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.

President Joe Biden has stood firm on his decision to end the US war but authorized the deployment of 3,000 troops to evacuate embassy staff and Afghan allies from Kabul.

The Pentagon acknowledged its concerns about the situation on the ground 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."