"And we have things under control, very well with a small force. We can probably make it a little bit smaller and then we'll decide. It'll depend on the Taliban. It will depend on the Afghan government.
"But there is a case to be made and the case also is that we're going to be leaving very significant intelligence behind for just the reasons I stated."
About 14,000 US troops remain in Afghanistan, training and advising Afghan security forces and conducting counter-insurgency operations.
"And we have a very very good view. I mean some things are going to be announced over the next couple of weeks as to what happened, who's been taken out. A lot of people have been taken out that were very bad, both IS and Al Qaeda," he added.
Both US negotiators and the Taliban have reported progress after eight rounds of talks since late last year.
Earlier this month, a senior Taliban member said that the US and negotiators of the militant group were likely to sign the much-awaited peace agreement.