The Trump administration allowed troops to engage in some law enforcement activities and potentially to use deadly force, according to a report by Military Times on Wednesday.

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Under Kelly's order, military personnel at the border are allowed to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary,” according to the report.

The "protective activities" include “a show or use of force (including lethal force, where necessary), crowd control, temporary detention, and cursory search.”

Senior administration officials said the order to expand the military's role at the border came from the president, but was conveyed by Kelly's memo. “The brave men and women at Customs Border and Protection willingly put themselves in extremely dangerous situations every day to protect Americans and their families. The President’s authorization ensures the Department of Defense can step in to protect those who protect us,” said White House Deputy Press Secretary Hogan Gidley in a statement.

Trump has so far deployed 5,900 active-duty service members to the border, along with 2,100 National Guard troops.

The orders allowing military personnel to engage in law enforcement activities are likely to be challenged in court, as activists have warned such a move would violate the 1878 Posse Comitatus Act.

That law bars active military from engaging in domestic law enforcement, although military force can be used on U.S. soil to "suppress insurrection or to enforce federal authority," according to a report by the Congressional Research Service.

The troops deployed to the border are tasked with defending Border Patrol and Customs and Border Protection officers from potential aggression by incoming migrants.

While military personnel always have a right to self-defense, the orders to defend third-party law enforcement agencies are unprecedented, according to the Military Times.

In his order, Kelly said the administration has intelligence indicating migrants currently in Tijuana, Mexico, “may prompt incidents of violence and disorder” at the border.

Thousands of migrants who crossed Mexico in a caravan, most of them Hondurans or Guatemalans, are currently massed in Tijuana.

Their arrival at the Mexican border city prompted demonstrations for or against the migrants, and a brawl over the weekend, but the caravan members have been relocated to shelters.

-- Updated at 5:35 p.m. ET