(CNN) Secretary of Defense Mark Esper has approved a request from the Department of Homeland Security to continue military support to the southern border but reduce the number of authorized troops by 1,500 with most of the 4,000 deployed drawn from the National Guard, the Pentagon announced Thursday.
"Secretary Esper has approved a request for assistance from the Department of Homeland Security and authorized the deployment of up to 4,000 DOD personnel to the southern border, beginning in October," Pentagon spokesman Lt. Col. Chris Mitchell said in a statement.
That marks a decrease from the current authorized troop level of 5,500.
"This DoD support will primarily be sourced from the National Guard," Mitchell added, saying that the National Guard would be federalized while performing this mission, meaning that they will be both funded and commanded by the federal government.
The announcement comes at a time of tension between Pentagon leadership and the White House after another controversial deployment of troops following the protests across the country, sparked by the killing of George Floyd.
Several current defense officials with access to Esper and chairman of the Joint Chiefs of Staff Mark Milley have told CNN they are both are deeply concerned about Trump attempting to politicize the military and potentially dragging the forces into the presidential election campaign.
CNN reported in April that the Defense Department was actively weighing reducing the number of active duty troops on the southern border with Mexico and replacing them with members of the National Guard, which would bring the deployment into line with previous military operations on the US-Mexico border under the Bush and Obama administrations.
Pentagon officials stressed that troops on the southern border would not be performing law enforcement missions.
"The duties to be performed by military personnel include the same categories of support as those currently being carried out along the border, including detection and monitoring, logistics, and transportation support to U.S. Customs and Border Protection. Military personnel will not directly participate in civilian law enforcement activities," Mitchell said.
The current border deployment is about half active duty and half national guard personnel.
Military forces have emplaced concertina wire and other barriers, transported Customs and Border Protection personnel, and conducted mobile surveillance along the border.
The border operation has been a major priority for President Donald Trump and has been criticized by Democrats in Congress.
Trump first ordered the deployment of some 5,000 active duty troops to the border in 2018 in response to the anticipated arrival of a migrant caravan from Central America. He had previously deployed 2,100 National Guard troops to the border in April of that year.