President Donald Trump. File picture: Evan Vucci/AP Photo.

WASHINGTON - U.S. President Donald Trump said on Thursday he was "bringing out the military" to protect the U.S. border as a caravan of Central American migrants continued to trek through Mexico to the United States.

"I am bringing out the military for this National Emergency. They will be stopped!" Trump wrote in a post on Twitter.

White House and Pentagon officials did not immediately respond to requests for comment about the military deployment or the national emergency.

On Monday, Trump said he had alerted "Border Patrol and Military" that the migrant caravan was a national emergency. But a Pentagon spokesman said at the time that the defense department had not been asked to provide additional support.

Trump promised before the 2016 presidential election to build a wall along the southern border between U.S. and Mexico, but funding for his signature campaign promise has been slow to materialise.

In April, frustrated by lack of progress on the wall, Trump ordered the National Guard to help secure the border in four southwestern states. 

There are currently 2 100 National Guard troops along the borders of Texas, New Mexico, Arizona and California.