(CNN) Acting US Defense Secretary Patrick Shanahan said, beginning Sunday, he will start studying which projects military money may come from to shift funds toward a border wall following President Donald Trump's national emergency declaration.

Trump on Friday declared a national emergency to secure federal funds to build a wall on the southern border, bypassing Congress after lawmakers refused to meet his multi-billion dollar request for border wall funds.

The administration has said $2.5 billion of military narcotics funding and $3.6 billion in military construction money will be diverted to the wall. But Shanahan will have final say on how much will be taken from which programs.

A military official said Shanahan is likely to approve the $3.6 billion figure.

"I think I have a lot of discretion," Shanahan told reporters on a flight from Munich to the US Saturday. "You can trust the numbers in terms of the potential. Then you gotta marry it up with where the money would be spent."

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