President Trump's plan to send National Guard troops to the Mexico border: Here's what we know – and what we don't know

President Donald Trump's order to deploy National Guard forces has left key national security players in his administration scrambling to nail down details for the mission, such as its scope, its cost and its duration.

Top officials, however, have said they have been in constant communication with each other regarding the plan and the overall border security strategy in general.

"Secretary [James] Mattis agrees with Secretary [Kirstjen] Nielsen that border security is national security," chief Pentagon spokeswoman Dana White said Thursday during a press briefing.

"The communication between the White House and the Defense Department is very clear," White said.

Building a border wall, and having Mexico pay for it, was one of Trump's key pitches to his nationalist voter base during the 2016 campaign. But since he hasn't been able to secure the funding he desires for the wall, approximately $25 billion, he is instead pushing for heightened military presence at the border.

"Until we can have a wall and proper security we're going to be guarding our border with the military. That's a big step," Trump said Tuesday at the White House.

"We are going to be doing things militarily," Trump added, saying that he discussed the idea with Defense Secretary James Mattis.

As of yet, U.S. officials have provided few logistical details for carrying out Trump's orders.

Here's what we know and don't know about the mission.

So far, the White House, Department of Homeland Security and Department of Defense have yet to give even a ballpark figure of how many troops will be sent to the southern border with Mexico.

Trump himself told reporters Thursday that there could be "anywhere from two to four thousand" National Guard members deployed. The president didn't offer a specific number when reporters asked him about the potential cost. "Depends on what we do," he said.

A senior administration official, who spoke on condition of anonymity, said this week that despite the lack of specific details the discussions with the Defense Department to secure the border with troops is by no means new.

"We are not going to DoD and saying we need X number of people; we are going to DoD and saying we need to fulfill this mission requirement, and DoD is working to identify and task that out," the official said.

White, the Pentagon spokeswoman, said Thursday that the department is establishing a new border security support cell to analyze and expedite the most appropriate way to deploy troops.

When asked for an estimate of how many National Guardsmen would move to the border, White said that that will be decided by the aforementioned cell.

Another item White noted, is that the Pentagon currently supports the DHS border security mission with efforts from NORTHCOM, SOUTHCOM, PACOM, the National Guard and the Army Corps of Engineers.

During Wednesday's White House press briefing Nielsen, of Homeland Security, announced that troops would deploy immediately, even going as far to say that the National Guard could mobilize as soon as that evening.

When asked what the expectation would be for troop movement to the border, White offered little to further the clarify the timeline saying "we are working out those details."

"Those conversations with the governors are going on, we also have to see the requirements and the missions and then that will better determine how we move forward and how quickly we move forward," White said.

U.S. Marine Corps Lt. Gen. Kenneth McKenzie, the director of the Joint Staff, was also on hand for the Pentagon briefing and added that department will move "very quickly" once the department is aware of the full scope of the requirement.

While the number of National Guard members and timeline for the mission are still unknown, the Pentagon has described some of the troops' responsibilities.

The National Guard will support the Custom and Border Protection with aviation, engineering, surveillance, communications, vehicle maintenance and logistical support.

"Our support to DHS also includes the use of DoD equipment and facilities as well as training," White said.

McKenzie added that the department will have a better understanding of what the mission will be once secretary Nielsen concludes discussions with the governors in the border states.

McKenzie said that he expects those conversations to wrap up soon.

"I don't have any specific details on what support we could provide beyond that which Dana has already outlined except to tell you that it will be guided by what the requirements are that are identified by the governors in consultation with DHS," he added.

It is also unclear whether the National Guard who are mobilized to the border will be armed.

"Those are exactly the questions that we are answering now, I don't have answers for you on any of those," McKenzie said in reference to various questions on whether troops would be allowed to carry weapons.

"Those are all good questions, we will answer each of those questions in great detail as we deploy," he added.

White noted that the troops will be supporting border patrol agents and those officers will be conducting law enforcement activities.