The top US general said Wednesday that it was not possible to predict Afghanistan's fate after the US troop withdrawal, warning of the "worst-case" outcome of a collapse of the government to the Taliban.

"Tough situation, no good answers to any of it," Joint Chiefs Chairman General Mark Milley told the Sedona Forum when asked about the future of Afghanistan.

He said that the Pentagon would withdraw its last 2,500-plus forces by September under the decision announced two weeks ago by President Joe Biden, to end the two-decade US military involvement there.

"What comes after that, I think there's a... range of outcomes, some of which are quite bad, some which are not quite bad," Milley said.

"On the worst-case analysis, you have a potential collapse of the government, a potential collapse of the military. You have a civil war and all the humanitarian catastrophe that goes with it," he said.

That could include the revival of Al-Qaeda, the jihadist group behind the 9/11 attacks that was the target of the original US-led invasion of the country in 2001.

"On the other hand, you do have an army, that's a 350,000-strong army and police forces and the Afghan security forces; you do have a government today," Milley said.

"They have been engaged in counterinsurgency operations for quite some time against the Taliban. So it's not a foregone conclusion that there'll be an automatic fall of Kabul, so to speak."

Milley said the best outcome would be a negotiated deal between the Kabul government and the Taliban rebels, but he declined to guess which of the possible paths the country would take after US troops leave.

He said that even after the US troop departure, they will still be able to monitor and pursue Al-Qaeda if the group seeks to expand its influence.

"We have a lot of capabilities...  in order to track and then target enemies of our country," Milley said.

"Afghanistan and Pakistan are not the only places that have these terrorist threats. So we're capable of monitoring, we're capable of striking if the need arises."