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(CNN) —  

The Trump administration is actively examining using billions of dollars in unspent Defense Department disaster recovery and military construction funds for the construction of a border wall in the event the President declares a national emergency, according to a US official.

Congress appropriated $14 billion in supplemental funds to repair infrastructure in areas of the country hardest hit by disasters including hurricanes, like Hurricane Maria which slammed Puerto Rico in 2017 and resulted in the deaths of nearly 3,000 people.

In anticipation of a national emergency declaration, the official tells CNN that the Pentagon was asked to provide lists of unspent funds including those earmarked for civil works projects that are part of disaster recovery in Puerto Rico, Texas, California, Florida, and elsewhere. The official said the funds were only recently received. There is more than $13 billion not yet physically spent on the infrastructure repair projects, but that have been promised to these communities.

For instance, more than $2 billion planned for projects in Puerto Rico has not yet been spent. More than $4.5 billion for projects in Texas, including those related to 2017’s Hurricane Harvey, has also not been spent.

Trump was briefed Thursday on a proposal to use $5 billion for portions of the wall based on priorities identified by the Department of Homeland Security, the official said. Under a current proposal, a 30-foot high steel slat wall – the so-called bollard wall – would be put up across 315 miles of federal lands over 18 months.

As the government shutdown continues with no deal between the White House and congressional Democrats in sight, President Trump inched closer to declaring a national emergency to approval to begin construction of the border wall.

“If this doesn’t work out, probably I will do it, I would almost say definitely,” Trump said at the White House. “This is a national emergency.”

“If we don’t make a deal, I would say 100%. I don’t want to say 100% because maybe something else comes up,” he said. “But if we don’t make a deal, I would say it would be very surprising to me that I would not declare a national emergency and just fund it through the various mechanisms.”

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Title 10 of the US Code allows Trump to unlock a stash of Pentagon funds that are earmarked but have no signed contracts for spending that money.

Anything beyond that would require the cancellation of existing military construction projects, which might come with costly termination fees.

If Trump wants additional funds from projects that are already in contract, he’d have to cancel projects like: a fire station at Quantico, child development at Joint Base Andrews, or Navy SEAL training facilities improvements for combat training.

One major concern for the Pentagon is that once money is diverted to the wall, it’s unclear when or if Congress would replenish funds for any canceled or delayed projects so that readiness is not impacted.

Within days of an emergency declaration by the President, personnel would begin to deploy to start the project, the official said. Construction of more than 80 miles of barrier could begin within 45 days along parts of the border in California and near El Paso, Texas. More difficult terrain and conditions mean sections in some of the highest priority areas for DHS and Customs and Border Protection in the Rio Grande Valley Sector would take longer to build, with some parts of those projects not starting for at least six months.

The official says the current plan would only apply to federally owned lands and the emergency declaration would not provide authority to build on private lands. There’s currently a variation of fencing along some 650 miles of the southern border.

No copy of a potential declaration has been made public.

The Pentagon has assisted DHS on the border wall in the past. For example, the Army Corps of Engineers, a federal agency within DOD that provides public engineering services, has helped evaluate prototypes of the border wall.

Trump has argued that an increase in border apprehensions necessitates a wall and additional reinforcements to stem the flow of migrants. According to federal data, Customs and Border Protection apprehended nearly 400,000 people along the southwest border in fiscal year 2018, an increase from fiscal year 2017, but a decrease from 2016.

Acting Secretary of Defense Patrick Shanahan participated in his first meeting as a member of Trump’s Cabinet Wednesday at the White House, where he briefly spoke about the Department of Defense’s support on the border.

“The Army Corps of Engineers is dialed in on doing this cost-effectively, quickly, and with the right amount of urgency as to where we can build additional standup walls quickly and then get after the threat,” Shanahan said. “The threat is real, the risks are real. We need to control our borders.”

Trump praised the Defense Department and Shanahan’s “fantastic” work, adding that people can’t get through the portions of the wall that have been constructed “unless you’re a world-class pole vaulter on the Olympic team.”

CNN’s Betsy Klein contributed to this report.