The deployment will raise the total active duty forces to approximately 4,350.
“Additional units are being deployed for 90 days, and we will continue to evaluate the force composition required to meet the mission to protect and secure the southern border,” the Pentagon said.
Roughly 2,400 active duty forces are currently at the border. A large number are going home and being replaced by the new deployment.
The announcement comes as a congressional committee seeks to reach a deal on border security funding to avert another partial government shutdown.
President Trump has hinted at declaring a national emergency if the deal doesn’t include funds for a border wall. The lack of an agreement on funding for a border wall closed portions of the government for 35 days.
Trump ordered the deployments in October 2018 in response to the impending arrival at the border of a caravan of asylum-seekers from Central America.
The new troops will be used to install additional wire barriers and provide a large new system of mobile surveillance and monitoring of the border area.
The Pentagon has approved an extended U.S. deployment to the border through the end of September.
Last week, members of the House Armed Services Committee grilled top defense and military leaders about the border mission, demanding details on its impact on military readiness and whether needed training and other jobs were not being done as a result.
“What impact does it have to readiness to send several thousand troops down to the Southern border? It interrupts their training. It interrupts their dwell time,” said Rep. Adam Smith, D-Wash., and chairman of the committee.
Fox News Pentagon producer Lucas Tomlinson and The Associated Press contributed to this report.