U.S. troops at the border have been authorized to use force, including "lethal force," according to a new White House memo -- a controversial expansion of military power by the Trump administration to crack down on illegal immigration. Secretary of Defense James Mattis, however, insisted Wednesday that troops won't use such force.

The memo, dated Tuesday and signed by White House chief of staff John Kelly, says troops at the border "may perform those military protective activities that the secretary of defense determines are reasonably necessary to ensure the protection of federal personnel, including a show or use of force (including lethal force, where necessary), crowd control, temporary detention, and cursory search."

"The deployed military personnel shall not, without further direction from you, conduct traditional civilian law enforcement activities, such as arrest, search and seizure in connection with the enforcement of the laws," says the memo, which was obtained by CBS News national security correspondent David Martin.

Get Breaking News Delivered to Your Inbox

The Military Times first reported that such a memo existed, and Newsweek later obtained the memo written by Kelly, and another one signed by Mr. Trump. There are currently more than 5,700 troops at the border.

Mattis confirmed to reporters Wednesday that Kelly sent him a letter Tuesday evening authorizing new authorities, but he said "there has been no call for any lethal force from DHS." Mattis said he is reviewing what instructions he will issue to troops on the border, but emphasized the the military "will not be armed, not with a firearm." Military police will be armed "with shields, not firearms," Mattis said, adding that troops "do not have arrest authority."

Mattis said that if troops do witness someone beating a border patrol agent, they would intervene and hold onto the culprits just long enough — "minutes" — to turn them over to law enforcement. Mattis also said "some of the troops" will be home by Christmas. The total cost so far for deploying thousands of troops to the border is $72 million, although Mattis said he's "confident" that number will increase.

"If an army unit along the border sees an agent in distress or is informed of one, the general view is that they can probably go to their aid to safeguard themselves or the agent," George Washington University law professor Jonathan Turley told CBS News' Sara Cook. "But in terms of the systemic use of the military like we have today, this is far more problematic, and runs contrary to statutory and constitutional norms. What the president has done would not normally be viewed as a legitimate use of the military. There have been no reports of agents at risk along the border. The use of the military to law down concertina wire and perform tasks gets dangerously close to law enforcement functions."

The White House eventually responded to requests for comment.

"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," deputy White House press secretary Hogan Gidley said in a statement.

The memo, signed shortly before the Thanksgiving holiday, represents the Trump administration's latest crackdown on border security and illegal immigration. Mr. Trump has fumed over immigration as a caravan of migrants approaches the southern border, and has, through the Pentagon, directed thousands of troops to support border personnel there.

Mr. Trump has appeared to make conflicting comments in the past as to whether he believes troops can use force at the border. In announcing a new asylum policy, the president said in early November that troops should treat any rocks thrown at them like "a rifle" and respond accordingly.

"We will consider that the maximum that we can consider that, because they're throwing rocks viciously and violently," he said. "You saw that three days ago. Really hurting the military. We're not going to put up with that. If they want to throw rocks at our military, our military fights back. We're going to consider — and I told them, consider it a rifle. When they throw rocks like they did at the Mexico military and police, I say, consider it a rifle."

But he also said he would hope that troops wouldn't use force, and acknowledged that the troops are to support border personnel already there.