Trump on Friday declared a national emergency to address what he called an "invasion" of drugs, gangs, human traffickers and undocumented migrants over the US border with Mexico.

The declaration means he would be able to sidestep Congress to access federal funds from elsewhere to help pay for building the border wall. The move however was immediatly challenged in court.

"So very deliberately, we have not made any decisions. ... There has been no determination by me," Shanahan said on the way home from Germany.

Trump wanted Congress to authorize $5.7 billion for a wall along parts of the border, but lawmakers provided just $1.375 billion for barriers, not a solid concrete wall.

The White House said Friday that Trump will access $6.1 billion from two Pentagon sources: $3.6 billion from a military construction fund, and $2.5 billion from Defense Department counter-drug activity funds.

"We always anticipated that this would create a lot of attention and since money potentially could be redirected, you can imagine the concerns this generates," Shanahan said.

The Pentagon chief said he would "review that analysis now that the emergency has been declared. Based on that analysis, we can do an assessment of what would be appropriate."

He added: "We are following the law. We are using the rules. We are not bending the rules."

As a candidate, Trump repeatedly vowed that Mexico would pay for a wall.