In a news conference on Monday, the top American commander in Afghanistan, Gen. Austin S. Miller, confirmed that the size of the American force in the country had already quietly dropped by 2,000 over the last year, down to roughly 12,000.

Other American and Afghan officials, speaking on condition of anonymity to discuss details of the plan, said that the eventual force size could drop to as low as 8,600 — roughly the size of an initial reduction envisioned in a draft agreement with the Taliban before Mr. Trump halted peace talks last month. Rather than a formal withdrawal order, they are reducing the force through a gradual process of not replacing troops as they cycle out.

A senior Afghan official said the Afghan government had signed off on the reduction. Officials would not discuss other details of the drawdown, including any specific timeline for it.