Speaking on condition of anonymity, the official said the troops would be used mainly to provide logistical support including tents, vehicles and equipment.

Trump earlier Thursday tweeted that "Democrat inspired" laws make it difficult to stop people at the border.

"I am bringing out the military for this National Emergency. They will be stopped!" he said.

About 2,100 National Guardsmen are already deployed to the border, following a Trump order in April.

Those troops are mainly serving in a support role to help free up border patrol officers.

CNN first reported the development, saying Defense Secretary Jim Mattis was expected to send 800 or more troops.

It was not immediately clear if the new deployment would be comprised of active-duty troops or additional Guardsmen.

The move comes as thousands of Central American migrants are crossing Mexico toward the United States in a caravan, drawing near-daily criticism from Trump.

Early Thursday, they set off from the town of Mapastepec in the southern Mexican state of Chiapas, moving on to the next stop in their long march north.

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 US border.

"It's hard, and we know this country is dangerous too, but back in Honduras it's even more dangerous, they kill for nothing," said Josena Anibal Mejia, 27, as he walked with his daughter.

The United Nations estimates that 7,000 people have joined the caravan since it set out from San Pedro Sula, Honduras October 13.