"At the request of Secretary of Defense Mattis, the president has delegated force management authority to the secretary," Pentagon spokesman Maj. Adrian Rankine-Galloway told CNN Wednesday.
A US senior defense official told CNN that the decision concerning troop levels was communicated via an internal policy memo April 20.
The current deployment numbers have not been changed.
"This delegation of authority does not change the force management levels for Iraq and Syria," Rankine-Galloway said, adding that the move "does not portend a change in our mission in Iraq and Syria to defeat ISIS."
"Our strategy remains to work by, with and through local forces and to conduct all operations in Iraq with the approval of the government of Iraq," he added.
He did say the change "enables military commanders to become more agile, adaptive and efficient in supporting our partners."
Troop specifications have historically served as a political marker to define the scale of US military involvement in places like Iraq, but military officials see the numbers as arbitrary, forcing military commanders to artificially split units and use contractors to meet the caps. The practice of designating personnel ceilings dates back at least to the Vietnam War, according to the Government Accountability Office.
Under the Obama administration, troop levels were set by the White House. Staff were deeply involved in setting its limits compared to previous administrations, according to multiple current and former officials, and made frequent adjustments. In December, the Syria troop levels were expanded to allow for 200 more troops.
The Pentagon characterized the change as a return to previous practice.
"The Secretary of Defense has historically maintained force management authority during contingency operations," Rankine-Galloway said.
"This is not new. This restores that authority and is a more effective way of managing combat power," he added.
The headcount can allow for host nations like Iraq to better understand the scale of the foreign troop presence.
But the troop specifications have been criticized for not adequately reflecting the real number of troops on the ground.
Buzzfeed was first to report the authority had been delegated to Mattis.
The troop levels for Syria are currently 503, but the number of troops on the ground is closer to 1,000, while in Iraq the amount is a little over 5,000, though the real number of troops is about 7,000 military, officials have told CNN in the past. Officials have said that the difference is due to temporary forces not being counted against the total.
The senior defense official CNN spoke to said that the Pentagon is currently reviewing the use of troop levels in order to determine how to better count US troops on the ground.