"At noon yesterday, President Trump delegated to me the authority to manage troop numbers in Afghanistan," Mattis told the Senate Appropriations Committee during a hearing on the Pentagon's budget request.
"The delegation of this authority, consistent with the authority President Trump granted me two months ago for Iraq and Syria, does not at this time change the troop numbers for Afghanistan," Mattis added.
"This assures the department can facilitate our missions and nimbly align our commitment to the situation on the ground," he said of the new authority.
"Our overall mission in Afghanistan remains the same, to train, advise and assist the Afghan forces so they can safeguard the Afghan people and terrorists can find no haven in Afghanistan for attacking us or others," Mattis said, saying the new strategy for Afghanistan will be presented to Trump in the "coming weeks."
This is a change from the Bush and Obama administrations, where the White House approved troop levels, largely because tens of thousands of personnel were involved.
A US senior defense official told CNN that the decision concerning troop levels in Syria and Iraq was communicated via an internal policy memo on April 20.
There are currently about 8,400 US troops devoted to Operation Resolute Support in Afghanistan, which encompasses both US counter terrorism forces to fight ISIS and the Taliban as well as the effort to train, advise and assist Afghan forces in a separate effort.
The Pentagon and White House have been reviewing an option to send 3,000 to 5,000 additional US forces solely in that training and advisory role.
The Pentagon is now conducting a broader military review involving both Afghanistan and Pakistan, so a decision on exactly how many additional troops might not come for several more weeks, defense officials have said in recent days.
The issue of troop levels became especially sensitive in the Obama administration because precise so-called "force management levels" were set for US deployments to Iraq and Syria. But those troop levels quickly became relatively meaningless because they were increased several times, and did not account for temporary deployments or the use of private contractors.