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(CNN) —  

The Defense Department is scouting sites along the US-Mexico border to erect new physical barriers, according to two department officials.

Small teams of engineers and experts, comprised of about 10 personnel, are on the ground in Yuma, Arizona, and the New Mexico part of the El Paso sector, which also includes Texas, looking at sites, the officials said.

Jay Field, a spokesperson for the US Army Corps of Engineers, later confirmed the teams’ presence on the ground, telling CNN that the eventual plan is to install 11 miles of fencing at Yuma and 46 miles at El Paso.

Each assessment should take about seven days. Formal engineering surveys are expected to follow in late April following the award of contracts to builders, which is expected to happen in mid-April.

Actual construction could begin by late May, depending on whether the Department of Homeland Security issues environmental waivers, which is sometimes done to expedite construction. The new construction will be funded by the $1 billion recently transferred by the Pentagon.

The departments of Defense and Homeland Security did not immediately return requests for comment.

RELATED: Almost 40,000 children will be taken into federal custody this month, US border official says

The move comes shortly after the Pentagon notified Congress that it authorized the transfer of $1 billion. The funds are set to go toward building 57 miles fencing along the southwest border, in addition to improving roads and other measures in the region.

The Army Corps of Engineers was allowed to begin planning and construction for the project Monday night. The department will direct the funds toward 18-foot-high fencing along the Yuma and El Paso sections of the border, according to a letter acting Secretary of Defense Patrick Shanahan sent to Secretary of Homeland Security Kirstjen Nielsen.

The administration has previously said it plans to shift an additional $1.5 billion at some point in the future.

In February, President Donald Trump declared a national emergency in order to funnel billions of dollars to wall construction. As part of his announcement, he directed the use of counterdrug monies to partially fund new wall construction. Under the declaration, other funds can also be dedicated to building the wall and related infrastructure, including military construction funds.

As the number of migrants crossing the border increase, Trump this week has renewed his attacks against congressional Democrats.

“We have a National Emergency at our Southern Border. The Dems refuse to do what they know is necessary - amend our immigration laws. Would immediately solve the problem! Mexico, with the strongest immigration laws in the World, refuses to help with illegal immigration & drugs!” Trump tweeted Thursday.

Trump’s national emergency declaration sparked a slew of lawsuits and pushback from Democrats and Republicans. On Monday, Senate Democrats objected the transfer of money to build fencing, saying in a letter to Shanahan that they “have serious concerns that the Department has allowed political interference and pet projects to come ahead of many near-term, critical readiness issues facing our military.”

Customs and Border Protection has already been working on updating its barriers.

According to CBP, the agency used fiscal year 2017 funds to construct “approximately 38 of 40 miles of wall in place of outdated designs in San Diego and El Centro, California, Santa Teresa, New Mexico and El Paso, Texas.”

A CBP official said “this barrier replaced dilapidated infrastructure” half of which was vehicle barrier designed to stop cars, but people can easily step over it.

CBP also previously said construction has “started on the San Diego Secondary replacement, the first project funded in FY 2018,” adding that “the first new wall project, where no barrier currently exists, is anticipated to start in April in Hidalgo County of the Rio Grande Valley Sector.”

The spending bill passed in February included $1.375 billion for approximately 55 miles of new barriers in the Rio Grande Valley sector.

CORRECTION: This story has been updated to reflect the location of personnel on the border.