Major Chris Mitchell, a Pentagon spokesman, told Reuters that acting Defense Secretary Richard Spencer had approved the additional troops on Tuesday night, and they would be assisting with tasks like logistical support and aerial surveillance.
There are currently about 4,500 active duty and National Guard troops on the border with Mexico.
The Pentagon first approved the high-profile deployment of active-duty U.S. troops to the Mexico border in October, before the November congressional elections. The move was embraced by Trump’s supporters, including fellow Republicans in Congress.
Critics called the deployment a political stunt.
Trump has made immigration a signature issue of his presidency. He declared a national emergency over the issue earlier this year in an effort to redirect funding from Congress to build a wall along the U.S. southern border.
The Trump administration on Monday unveiled a new rule to bar almost all immigrants from applying for asylum at the southern border, requiring them to first pursue safe haven in a third country through which they had traveled on the way to the United States.
Last month, Mexico has deployed almost 15,000 soldiers and National Guard in the north of the country to stem the flow of illegal immigration across the border into the United States.