WASHINGTON — Secretary of Defense James Mattis now has the authority to direct active duty military at the border to protect Customs and Border Protection officials with lethal force, and military police deployed to the border are training with riot gear, according to two military officials.

NBC News obtained a copy of a memo from White House Chief of Staff John Kelly, dated Tuesday, which describes the need for "Department of Defense military personnel" to temporarily protect CBP "by protecting their performance of Federal functions."

"To carry out that mission," the memo says, "these deployed Department of Defense military personnel 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."

Secretary Mattis acknowledged receiving the new authority but said repeatedly Wednesday that the Department of Homeland Security has not asked the Pentagon to take on any new security mission and the role of U.S. troops along the border has not changed.

"There has been no call for any lethal force from DHS," he told reporters at the Pentagon Wednesday morning.

Asked if troops on the border now have authority to use lethal force, Mattis said, "I have the authority. But we are not even employing, you've seen the picture of the guys down there not carrying guns, so relax. Don't worry about it."

Asked whether the U.S. military could be called in to back up CBP if things get rough, Mattis said, yes and that would likely include unarmed Military Police with masks, shields, and batons, but "no firearms."

Two U.S. military officials said military police deployed to the border are already training with their riot gear and that there are plans to send more MPs with riot gear to the border in California.

But Mattis emphasized that troops at the border will not have any law enforcement role, and that any possible detention of migrants would be extremely short.

"Detention would — I would put it in terms of minutes," he said. "In other words, if someone's beating on a Border Patrolman and if we were in position to have to do something about it, we could stop them from beating on them and take him over and deliver him to a Border Patrolman, who would then arrest him for it."

Kelly's memo lays out the potential threat along the border writing, "Credible evidence and intelligence indicate that migrant caravans originating from Central America and moving toward the southern border ... may prompt incidents of violence and disorder that could threaten U.S. Customs and Border Protection and other United States Government personnel."

Mattis would not put a timeline or final cost on the deployment, saying the $72 million figure the Pentagon announced on Tuesday was not a grand total. "I am confident that number will go up," he said.

He also left open the possibility U.S. troops would still be assigned to the border mission after the current end date on December 15. Asked whether troops could still be serving there on Christmas, Mattis said, "Some of those troops certainly will be home. I would anticipate they would be. But some troops may not be, or some new troops may be assigned to new missions. But this is a dynamic situation."

There are currently 5,764 active dury and 2,100 National Guard troops assigned to the border mission.