US military on border would have limited role President Trump wants to deploy the military along the U.S.-Mexico border.

President Donald Trump said Tuesday afternoon he wants to deploy the U.S. military to protect the border with Mexico while the new border wall is built.

Tuesday evening, White House press secretary Sarah Sanders issued a statement saying the "vigorous" administration strategy includes mobilization of the National Guard.

The White House statement came after Trump met with Defense Secretary James Mattis, Homeland Security Secretary Kirstjen Neilsen, Attorney General Jeff Sessions, Joint Chiefs Chairman Joseph Dunford and chief of staff John Kelly, Sanders said.

It is still unclear exactly what options were presented to the president by the Pentagon. However, by U.S. law the military would be limited to a support role for U.S. border patrol agents.

Earlier in the day, Trump said, "I've been speaking with Mattis, doing things militarily until we can have a wall and proper security. Already guarding our border with the military. That's a big step. We really haven't done that before, certainly not very much before."

"We are going to be doing some things — I've been speaking with (Defense Secretary) Mattis — we're going to be doing things militarily", Trump said at a White House meeting with the leaders of Baltic countries.

"Until we can have a wall and proper security, we're going to be guarding our border with the military," Trump said. "That's a big step. We really haven't done that before, or certainly not very much before."

"We are preparing for the military to secure our border between Mexico and the United States," Trump said.

Mexico's Ambassador Gerónimo Gutiérrez wants Trump to clarify his comments.

“We have formally requested a clarification on the President's comments from the State Department and the Department of Homeland Security," Gutiérrez said in a statement. "We share the idea of having a secure border, but we do not always agree on how to achieve that goal. Mexico, naturally, will always act in accordance with our own interests."

But while President Trump could order active duty military personnel or federalized National Guard troops to assist with border security operations, they would not be allowed to carry out law enforcement duties like detaining migrants crossing the border illegally.

That's because the Posse Comitatus Act prohibits the U.S. military from conducting law enforcement duties in the United States. The only way that can be allowed is by a special congressional authorization.

Trump's suggestion of U.S. military troops along the border with Mexico echoes actions by President George W. Bush in 2006 and by President Barack Obama in 2010 that sent thousands of National Guard troops to assist with border operations.

But the troops participating in Operation Jump Start and Operation Phalanx respectively were only allowed to serve in a support role for U.S. Customs and Border Protection (CBP) personnel.

In line with Posse Comitatus, they were not allowed to detain people crossing the border illegally from Mexico.

Instead, the thousands of National Guardsmen who participated in the earlier operations were used in mostly administrative and surveillance roles.

Guardsmen monitored video surveillance feeds from drones and stationary cameras along the border and manned watch towers located near the border from which they could notify border agents about illegal crossings.

Their presence was intended to free up CBP personnel that could be used in greater numbers along the border.