McKenzie repeated US military concerns that the withdrawal could allow the Taliban to advance, after President Joe Biden last week announced all American forces would pull out before this year's 20th anniversary of the September 11 attacks.

"We will bring additional resources in, in order to protect the force as it comes out," McKenzie told a US Senate hearing, giving no further details.

"That's normal in any kind of disengagement. I'm confident that... we will be able to extract ourselves."

Some 2,500 US troops, plus 16,000 civilian contractors and their equipment, must be evacuated from the country, along with some 7,000 NATO soldiers who depend on the US for transport.

The logistical operation will take at least three months to be completed safely, according to the US military.

Despite billions of dollars of investment, equipment and training, "my concern is the ability of the Afghan military to hold the ground that they're on now, without the support that they've been used to for many years," McKenzie said.

"It's intelligence, it's fire support, it's the enabling things that actually give them an edge over the Taliban. And all that will be gone."

He added "the ability of the Afghan Air Force to fly" without support was a key concern.

McKenzie dismissed talk of defeat, but he said the Taliban were more numerous than in 2011, estimating they have 50,000 fighters, and that they control more Afghan territory than 10 years ago.