In a statement sent out Thursday afternoon, Border Patrol says media groups, and possibly armed civilians, will be trying to access people's land without permission.
The additional troops are to provide what one official described as logistical support to the Border Patrol.
Arizona Gov. Doug Ducey sent hundreds of troops to the border earlier this year as part of "Operation Guardian Shield", a mission that extended through September 30.
The Trump administration is considering an order to block the entry of migrants at the southern border, using the same authority as its earlier travel ban, a person familiar with the matter said, as thousands of Central Americans make their way towards the United States. "I am bringing out the military for this National Emergency".
"I must, in the strongest of terms, ask Mexico to stop this onslaught", Trump tweeted. Earlier this year Mattis authorized Pentagon funding for up to 4,000 National Guard troops on the border and thus far only a little over 2,000 have been used. It also allows for the cost of deploying Guardsmen to be reimbursed through federal funds. Those troops are under the control of state governors and are limited to support roles such as providing surveillance. The Homeland Security Department disputed Judd's characterization, and said the Guard's assistance helped increase apprehensions and seizures of smuggled drugs.
Hundreds in migrant caravan turn back, some trickle into Mexico
More than 3,400 Hondurans migrants were returned to their home country over the weekend, according to a migrant assistance body. Mexican police in riot gear shadowed the caravan's arrival along a southern highway but did not impede the migrants' journey.
The president first threatened to send troops to the U.S. -Mexico border in April, telling foreign leaders that America would be "guarding our border with our military". Many are fleeing violence, poverty and government corruption in their home countries. Many said the fear of returning to a violent homeland loomed larger than the president's threats.
One piece of the action under consideration would use an authority similar to that invoked by the travel ban, which would block entry to unauthorized immigrants who are not yet in the US, said the three sources.
"To those in the Caravan, turnaround, we are not letting people into the United States illegally", the president wrote Thursday on Twitter. "Go back to your Country and if you want, apply for citizenship like millions of others are doing!"
The members of the 14th Caravan of Missing MIgrants' Mothers arrived at the Mexico-Guatemala border location of El Carmen-Talisman yesterday at noon, carrying pictures of their families and flags from Guatemala, Honduras, El Salvador and Nicaragua.