Active duty military troops will begin arriving in El Paso, Texas, and San Diego on Thursday evening, in what defense officials said is the first batch of the new forces being deployed to secure the southern border.

Troops have assisted at the border before and will be limited to support roles unless the administration invokes the Insurrection Act.

The US-Mexico border is effectively closed off to migrants seeking asylum in the United States within hours of President Donald Trump taking office, an extraordinary departure from previous protocols that has left many concerned migrants in limbo.

U.S. border agents have been instructed to summarily deport migrants crossing into the country illegally without allowing them to request legal protection.