Mr. Mattis said he would begin to outline the rationale for the forces in more detail when he testifies to Congress on Wednesday.
The revised troop figures, announced by the Pentagon on Monday, were the result of a new method of accounting for US soldiers, sailors, airmen and Marines serving in Afghanistan, Pentagon press secretary Dana White said Monday.
Pentagon chief declines to comment on number of additional troops but reports say the figure could be as high as 4,000.
"We had to change how they were accounting for them, because there were so many different pockets", Mattis said earlier in August.
"Under the new, simplified accounting methodology, the current total forces number in Afghanistan is approximately 11,000", he said. To date, more than 2,300 Americans have been killed and over 17,000 wounded in Afghanistan.
The administration has not announced how many troops are expected to deploy to Afghanistan. Almost 17 years after the US first invaded to dislodge the Islamic government of the Taliban, which refused to give up Al-Qaeda chief Osama bin Laden in the wake of the 9/11 attacks, Trump said he would no longer focus on nation building.
At its peak, the US military had some 100,000 troops in the country before scaling back and handing active combat operations over to Afghan forces.
There have always been political sensitivities within the Iraq government about the number of American troops on the ground, and those concerns raise questions about whether the Pentagon will be less candid about force numbers there to avoid conflicts. He did not provide numbers, however for troop in Iraq and Syria, saying they were "under review".
Previously, Defense Department officials had said 8,400 troops were in Afghanistan as part of NATO's Resolute Support mission.
Admitting the United States has more forces in Iraq than it now claims could be a liability for al-Abadi with Iraqis who are concerned about the United States' continued presence in the country, wrote John Glaser, the director of foreign policy studies at the Cato Institute, a libertarian think tank. Taliban has been carrying out deadly attacks in the country, including capital Kabul.