"There are roughly 3,600 personnel supporting that mission, and that number will be there through the end of the FY [fiscal year]," Mitchell said in a statement on Friday.
The troops will provide logistical support to the Department of Homeland Security and Customs and Border Protection as they work to secure the US southern border.
On Thursday, the White House said the US President Joe Biden had informed Congress that he had terminated the Trump administration's national emergency at the US southern border.
Defense Department spokesperson John Kirby told reporters that US troops remain on the border because their deployment was already funded by the previous administration.
Between October 2020 and January of this year, over 70,000 illegal migrants have been apprehended on the US-Mexico border. The number of undocumented immigrants apprehended has been increasing since April 2020. The last time the number of apprehensions peaked was in May 2019, with 144,116 border detentions.