In late August, Trump announced a South Asia strategy focused on ending the stalemate against the Taliban in Afghanistan.
Part of that strategy included an unspecified buildup of American forces to broaden the advise-and-assist mission to the Afghan military.
But Mattis held off on sending additional troops to Afghanistan as he awaited a presidential strategy decision for Afghanistan.
Following that decision, Mattis requested from military planners an updated recommendation based on the new strategy.
Reuters was first to report on Wednesday that about 3,500 additional American troops would be headed to Afghanistan.
On Friday, Mattis told reporters that he had signed deployment orders for some of the additional troops that would be sent, though he would not disclose the number.
On Wednesday, Mattis, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, the Secretary of State Rex Tillerson and Director of National Intelligence Dan Coats briefed members of Congress about the new strategy in Afghanistan.
Last week, the Pentagon disclosed that the number of American troops actually serving in Afghanistan was 11,000 and not the 8,400 official number it had been providing for some time.
The discrepancy was that only troops serving more than six-month tours in Afghanistan were being counted toward the official figure.