The so-called "border security support cell" will channel information between the Pentagon and the Department of Homeland Security, which is responsible for border security.

"The cell will last for the foreseeable future, to ensure we surge our capacity to meet the president's enhanced border security goals," Pentagon spokeswoman Dana White told reporters.

Trump on Wednesday ordered the National Guard to deploy to the southern US border, ratcheting up pressure on Mexico and taking another step in his quest to clamp down on illegal immigration.

Because the National Guard falls under the purview of state governors, the Pentagon needs to liaise with each of the four states bordering Mexico to determine how the military will operate.

"The National Guard's efforts will include aviation, engineering, surveillance, communications, vehicle maintenance and logistical support," White said.

But Pentagon officials still do not know how many troops will be on the border, whether they will be armed, when they will deploy and how long they will be there.

Pentagon planners have been scrambling to find ways to support Trump's surprise edict that he would send the military to the border.

The commander-in-chief's seemingly off-the-cuff directive blindsided officials Tuesday, when Trump said the military would guard the frontier until "we can have a wall and proper security."

White denied there was a lack of coordination or communication between Trump and the Pentagon.

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

Trump wants to build a wall along part of the Mexican border, but his pet project is facing financial hurdles and he has suggested poaching Pentagon cash to help pay for it.

White said the Defense Department is looking into building a stretch of wall at its Barry M. Goldwater Air Force Range in Arizona, where it abuts the border.