In an interaction with Pentagon reporters, Mattis, however, refused to give any details of the new troop levels in terms of their numbers, deployment and where they are coming from.
"Yes. I've signed orders but it's not completed. In other words, I've signed some of the troops that will go and we're identifying the specific ones," Mattis said.
Mattis' announcement came a day after the Pentagon disclosed that there are about 11,000 US troops in Afghanistan as against 8,600 announced earlier.
Unveiling his South Asia strategy ten days ago to win the war on terrorism, Trump said he will prolong the US military intervention in Afghanistan, which he had once described as a "complete waste".
Trump acknowledged that his "instincts" told him to pull out troops from Afghanistan.
"Historically, I like following my instincts," Trump said. "A hasty withdrawal," Trump said, would create a "vacuum" for terrorists.