Washington (CNN) Defense Secretary James Mattis is expected to sign deployment orders as soon as Thursday that could send 800 or more troops to the border with Mexico to help border patrol authorities stop a caravan of migrants from Central America moving through Mexico to enter the US, according to three administration officials.

The officials tell CNN final details are being worked out including where the troops will come from and their specific tasks.

Several administration officials say it is anticipated that the Pentagon will pay for the bulk of the mission by using active duty troops. It's not yet clear if the Department of Homeland Security will reimburse the Pentagon.

Officials also emphasize because the troops are there to support civil authorities, it is not expected they will come into any regular contact with migrants. If the troops carry arms, it will be solely for self-defense.

These officials say the Pentagon has been under growing pressure from the White House to send troops to the southern border, especially after the President repeatedly called for the use of the military. But the deployment is strongly expected to adhere to current law which prohibits the routine use of troops in law enforcement functions.

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