According to the Associated Press, Mattis is expected to send at least 800 troops to the border as a caravan of thousands of migrants seeking asylum in the US make their way north.
President Trump has vowed to "call up the U.S. Military and CLOSE OUR SOUTHERN BORDER" if Mexico is unable to stop a caravan of thousands of migrants from attempting to cross illegally into the United States.
The additional troops would provide logistical and other support to the Border Patrol, said the US official, who spoke on condition of anonymity to discuss a plan that had not been finalized and formally announced. They would assist the Border Patrol by providing things such as vehicles, tents and equipment.
Immigration and customs agents in the area have already been bolstered by some 2,100 National Guard troops, as the Trump administration has made it a priority to prevent undocumented immigrants from entering the U.S.
The official said the troops would satisfy elements of a "wish list" for military assistance sent to the Pentagon by the Department of Homeland Security, the USA agency with responsibility for the border. As the migrant caravan from Honduras approaches the US southern border, the President has just one message: "turnaround". The action comes as President Donald Trump has spent recent days calling attention to the caravan of Central Americans slowly making its way by foot into southern Mexico but still more than 1,000 miles (1,600 kilometers) from USA soil. Most are Hondurans, seeking to escape the poverty and violence that plagues the region. "You're going to see a very secure border", he said.
"It's hard, and we know this country is unsafe too, but back in Honduras it's even more risky, they kill for nothing", said Josena Anibal Mejia, 27, as he walked with his daughter.
The president said nobody is getting across our border illegally and the caravan should just turnaround now, in a tweet Thursday.
Guatemalan Foreign Minister Sandra Jovel dismissed the threat as a posture adopted for the media, telling reporters that the country had received no official notice of the move.
They were headed to the town of Pijijiapan, some on trucks but most making the seven-hour trek on foot. Four days after crossing into Mexico, the caravan is still more than 3,000 kilometers (1,860 miles) from the U.S. border.
"It's hard, and we know this country is risky too, but back in Honduras it's even more unsafe, they kill for nothing", said Josena Anibal Mejia, 27, as he walked with his daughter.