The US military could slow down its withdrawal from Afghanistan due to the gains made by the Taliban insurgents, Pentagon spokesman John Kirby said Monday.

Kirby stressed that President Joe Biden's deadline of a full withdrawal by September remains in place, but added that the pace could be adjusted based on conditions.

"The situation in Afghanistan changes as the Taliban continue to conduct these attacks and to raid district centres as well as the violence, which is still too high," he told reporters.

"If there needs to be changes made to the pace, or to the scope and scale of the retrograde, on any given day or in any given week, we want to maintain the flexibility to do that," he said.

"We're constantly taking a look at this, every single day: what's the situation on the ground, what capabilities do we have, what additional resources do we need to move out of Afghanistan and at what pace." "All of these decisions are literally being made in real time," he added.

Pentagon officials said last week that the withdrawal, ordered by Biden in April after nearly two decades fighting Al-Qaeda and helping government forces battle the Taliban, is around half completed.

At the time of Biden's order around 2,500 US troops and 16,000 contractors, mostly US citizens, were in the country. The Pentagon has already turned over several of its key bases to government security forces, and has removed hundreds of cargo plane-loads of equipment.

Kirby said US forces continue to support Afghan troops in fighting the Taliban.

"So long as we have the capability in Afghanistan, we will continue to provide assistance to Afghan forces," he said. "But as the retrograde gets closer to completion, those capabilities will wane and will no longer be available."