US Defence Secretary Lloyd Austin told Congress on Thursday that an extremist group like Al Qaeda or ISIS could regenerate in Afghanistan and pose a threat to America within two years of the US troop withdrawal.
Testifying at a Senate Appropriations Committee hearing, Mr Austin and Gen Mark Milley, the chairman of the Joint Chiefs of Staff, were asked if they rated such an outcome as small, medium or large.
“I would assess it as medium,” Mr Austin said.
Gen Milley said a collapse of the Afghan security forces would only increase that likelihood.
"There's a range of outcomes here," Gen Milley said, estimating the Kabul government now has about a 325,000 to 350,000 person military and police force.
"The question remains what will happen in the future," he said.
Gen Milley added the US is working to prevent an outcome where the Kabul government collapses but cautioned "there are no guarantees in any of this".
Asked about stalled efforts to grant US visas to the more than 18,000 Afghans who have worked with Washington since the invasion in 2001, Gen Milley assured Congress that planning is under way.
Jeanne Shaheen, a senator from New Hampshire, expressed concern the Afghanistan withdrawal could end like in Vietnam in the 1970s, when the US retreat preceded sparked a refugee crisis, with almost 800,000 Vietnamese fleeing the country in the years after.
Gen Milley, however, said he did not share the concern. "I don’t see Saigon 1975 in Afghanistan,” he said.
The Pentagon said last week that the US withdrawal from America’s longest war is a little more than half completed and on track to be fulfilled by the September 11 deadline.
The US and ally nations invaded Afghanistan after the 2001 terror attacks on America, when the Taliban allowed Al Qaeda safe haven in the country.
The key goal of US and coalition troops in Afghanistan since then has been to prevent a resurgence and another attack against America or other allies.