The President also warned officials could shut down the entire border to prevent migrants from Central America illegally crossing into the US. Mr Trump has stationed more than 6,000 troops along the border. He said: “If they have to they are going to use lethal force. I’ve given the OK – I hope they don’t have to. We’ll close the border if we find that it gets to a level where we are going to lose control or our people are going to start getting hurt.”
US Defence Secretary Jim Mattis said he had been granted authority to allow troops on the Mexican border greater powers to help protect border guards and would await direction from the Department of Homeland Security.
Mr Mattis said troops could help protect the border agents with shields and batons but would be unarmed.
Officials said a memo signed by White House Chief of Staff John Kelly gave Mr Mattis the authority to protect immigration agents and could include actions such as the use of lethal force, crowd control, temporary detention and cursory searches.
The order applies to 5,900 active-duty service members currently deployed to the region alongside 2,100 National Guard troops.
A caravan of almost 3,000 Central American migrants has arrived at the Mexican border city of Tijuana.
They say they are fleeing persecution, poverty and violence in their home countries of Honduras, Guatemala, and El Salvador.
But Mr Trump has described the caravan as an “invasion” and deployed the troops along the southern border to “harden” it.
The Pentagon has said in the past there were no plans for US forces to interact with migrants and that they have been carrying out support tasks.
Asked if the border mission could be extended, Mattis said it was dependent on the situation, but at least some troops would be back home before Christmas.
He also said that fresh troops could be assigned new missions.
The mission is currently authorised to run until December 15.