Acting US Defense Secretary Patrick Shanahan says he hasn't yet determined whether a border wall with Mexico was a military necessity or how much Pentagon money would be used.
President Donald Trump on Friday declared a national emergency in a bid to fund his promised wall at the US-Mexico border without congressional approval.
By declaring a national emergency, Trump can use certain Department of Defense funding to build the wall.
According to the law, the defense secretary has to decide whether the wall is militarily necessary before money from the military construction budget can be used.
"We always anticipated that this would create a lot of attention and since moneys potentially could be redirected, you can imagine the concern this generates," Shanahan told reporters travelling back with him from his trip to Afghanistan, the Middle East and Europe.
"Very deliberately, we have not made any decisions, we have identified the steps we would take to make those decisions," Shanahan said.
He added that military planners had done the initial analysis and he would start reviewing it on Sunday.
Officials have said that the administration had found nearly $US7 billion ($A9.8 billion) to reallocate to the wall, including about $US3.6 billion from the military construction budget and $US2.5 billion from a Defense Department drug interdiction fund.
A US defense official, speaking on the condition of anonymity, said Shanahan would meet with the service secretaries in the coming days to pick which specific projects the money should come from.
The official said that based on the initial analysis, Shanahan was likely to approve the $3.6 billion being redirected from the military construction budget.
The Republican president's move, circumventing Congress, seeks to make good on a 2016 presidential campaign pledge to build a border wall that Trump insists is necessary to curtail illegal immigration.
Within hours, the action was challenged in a lawsuit filed on behalf of three Texas landowners.
"We are following the law, using the rules and we're not bending the rules," Shanahan said.
Australian Associated Press