The Pentagon is sending 3,750 US forces to the southwest border with Mexico for three months to provide additional support to border agents, the Department of Defence said on Sunday.
This will raise the total number of active-duty forces supporting Customs and Border Protection agents there to approximately 4,350, it said.
The Pentagon first approved the stationing of active-duty US troops at to the Mexico border in October, before the country's midterm congressional elections. The move was embraced by President Donald Trump's supporters, including Republicans in Congress.
Acting Defence Secretary Patrick Shanahan gave the approval on January 11. US Representative Adam Smith, chairman of the House of Representatives Armed Services Committee, first disclosed plans to send more troops on Thursday.
Mr Smith criticised senior Pentagon officials for failing to disclose the information during a hearing before his committee in Congress on the issue two days earlier.