“It is exactly over 3,000 somewhat and frankly I haven’t signed the last of the orders right now as we look at specific, small elements that are going,” Mattis told reporters.
The extra troops would bring the total number of U.S. forces in Afghanistan to more than 14,000.
Mattis added most of the extra troops were already en route to Afghanistan or had been notified of their deployment.
The announcement follows President Trump’s South Asia strategy, announced in August, which aims to beef up U.S. forces in Afghanistan from the current 11,000 number.
The extra troops would bolster the Afghan military in its fight against the Taliban, al Qaeda and the Islamic State in Iraq and Syria.
Mattis in August revealed he had signed orders to send additional troops to Afghanistan but did not specify the size of the force.
Previous reports indicated Mattis would send closer to 3,500 additional troops, but he has closely kept the final number under wraps.