Kirby stressed that US 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 centers,” he told reporters.
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.
Meanwhile, Kirby claimed 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, adding that "As the retrograde gets closer to completion, those capabilities will wane and will no longer be available."