"We're going down to 8,600 and then we make a determination from there as to what happens ... we're bringing it down," Trump told Fox News Radio, again repeating that the US "could win that war so fast if I wanted to kill ten million people there ... which I don't."
Trump met with top national security advisers earlier this month at his golf course in Bedminster, New Jersey, to review a U.S.-Taliban peace plan.
The US currently has about 14,000 service members in Afghanistan, alongside NATO troops, helping to train and advise Afghan troops and conducting counterterrorism operations. The peace plan is expected to formalize a significant withdrawal of U.S. forces after nearly 18 years of war.
But Trump stressed that the U.S. will maintain a troop presence in Afghanistan despite the withdrawal.
"You have to keep a presence," he said Thursday, adding that the U.S. would be reducing troops "very substantially."
Trump also repeated a previous talking point, calling Afghanistan the "Harvard University of Terrorism" given its "rough, mountainous terrain."
On Wednesday, the top U.S. general said he is "not using the 'withdraw' word right now" in regards to Afghanistan, as U.S. and Taliban negotiators reportedly near a deal.
Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said any agreement with the Taliban must ensure that Afghanistan does not become a "sanctuary" for extremists. "I think it's premature, I'm not using the 'withdraw' word right now," he said. "We're going to make sure our, that Afghanistan's not a sanctuary."
CNN's Nicole Gaouette and Ryan Browne contributed reporting.
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