President Trump salutes as he walks off of Marine One on the South Lawn of the White House in Washington on April 5 after returning from a trip to West Virginia. (Susan Walsh/AP)

President Trump said Thursday that he wants to send 2,000 to 4,000 National Guard troops to the southern border as part of his administration’s effort to ramp up security there.

It was the first time the administration put numbers on the scope of the new plan to dispatch National Guard personnel to the border, which Trump first disclosed this week.

Speaking with reporters on Air Force One after an event in West Virginia, Trump said the administration is “looking” at the cost of the effort. As for how long the troops will be stationed there, Trump said that “we’ll probably keep them or a large portion of them” until a border wall is constructed.

Trump signed a presidential proclamation Wednesday directing the Department of Homeland Security and the Pentagon to coordinate with border-state governors to post National Guard troops along the border. But many of the specifics have yet to be worked out with the states. One such state — California — has been noncommittal about whether it would cooperate with the administration in the effort.

Homeland Security Secretary Kirstjen Nielsen has said that the administration hopes to send National Guard troops “immediately.” On Thursday, Pentagon officials declined to provide details on the size of the planned mission or when it would begin, saying those matters would be addressed once the Defense Department receives a more specific mandate from DHS.

In the meantime, Pentagon spokeswoman Dana White said the department had set up a “border security support cell” comprising a small number of Pentagon personnel that would seek to coordinate with DHS.

White said Guard activities would include “aviation, engineering, surveillance, communications, vehicle maintenance and logistical support,” adding that the Guard personnel would operate in support of border agents as the agents perform law enforcement duties.

“Secretary [Jim] Mattis agrees with Secretary Nielsen that border security is national security,” she told reporters at the Pentagon.

Lt. Gen. Kenneth McKenzie, director of the Joint Staff, said it was not yet clear whether the Guard personnel would be armed during their border duties, but added: “Our advice is always that soldiers are able to defend themselves, and they have the right of self-defense. However, we’re going to be guided by the dialogue that’s going on now” between government agencies.

But Mexican officials said late Wednesday that Nielsen told Mexico’s secretary of foreign affairs that the National Guard troops would not carry weapons. The Mexican foreign ministry said in a statement that Nielsen also told the Mexican secretary of foreign affairs, Luis Videgaray, that the troops will be in support mode and will not carry out immigration or customs duties.

“In all of its communications on the topic, the Mexican government has stated to the U.S. government that if the announced deployment of the National Guard results in the militarization of the border, it would seriously damage the bilateral relationship,” the Mexican foreign ministry said. “It has also reiterated the fundamental importance of respecting the dignity and human rights of the Mexicans in the United States.”

A DHS official said the department is “still evaluating and discussing with the border states the missions the Guardsmen will perform in support of federal law enforcement.”

“Decisions about equipment carried by Guardsmen — including firearms — is dependent on their assigned missions and will be made in conjunction with the Department of Defense and the border governors,” said the official, who spoke only on the condition that they not be named.