Trump has refused to concede his election loss to Democrat Joe Biden, who takes office Jan. 20, just five days after the troop withdrawals are to finish.
Miller, who refused to take questions from reporters after reading a prepared statement before TV cameras at the Pentagon, said the U.S. will reduce troop levels in Afghanistan from more than 4,500 to 2,500, and in Iraq from about 3,000 to 2,500.
It also runs counter to his guidance that troop withdrawals be based on the conditions on the ground, not a date on the calendar.
He has accused his predecessor, Barack Obama, of setting a timetable for troop withdrawals in Iraq and Afghanistan that worked against the achievement of military goals.
Biden has sounded less absolute about troop withdrawal.