U.S. troops on the border with Mexico are expected to get authority from President Donald Trump to protect Customs and Border Protection personnel if they come under assault by migrants, two defense officials said.

The force protection mandate, which didn't exist before, was requested by the Department of Homeland Security and could go into effect Tuesday, the officials told NBC News.

“As Secretary Nielsen has said, we will not allow our frontline personnel to be in harm’s way. We will do everything we can to protect those who defend our nation’s sovereignty and secure our border. We appreciate the Department of Defense stepping in to assist the Department of Homeland Security as needed," said DHS spokeswoman Katie Waldman.

About 5,800 troops have been deployed to the border to join the 2,000 already stationed there. They are a mix of active duty, reserve and National Guard forces and in addition to thousands of border patrol agents already deployed to the area.

The operation, renamed earlier this month from "Operation Faithful Patriot" to "border support," will cost at minimum $200 million, estimates say.

Officials familiar with the plans told NBC News last week that the troops would probably find themselves with little to do. For the most part, without authority to guard CBP personnel, they are laying barbed wire barriers and erecting tent facilities for themselves and the Border Patrol.

Trump has floated sending as many as 15,000 troops to the area.