Construction on a new border wall in Texas where none previously existed has begun along the Rio Grande river, U.S. Customs and Border Protection (CBP) confirmed to Fox News on Friday.

While 75 miles of wall has already been built under the Trump administration, until now all of those structures replaced existing ones such as vehicle barriers or dilapidated fences, which were ineffective before being replaced, according to the agency.

The brand-new wall is under construction southeast of McAllen in Texas’s Rio Grande Valley sector, which sees the most illegal crossings of any border sector, with more than 325,000 people apprehended there through August this year, according to CBP.

TRUMP ADMINISTRATION ENTERS NEW PHASE FOR BORDER WALL, SETS AMBITIOUS TIMETABLE AFTER SECURING LAND

“Rio Grande Valley is by far our busiest sector, so getting this new wall system capability down there for [the agents] is truly a game-changer," a CBP spokesman said. "Just last week, the agents on the frontlines told us the wall is something they need."

The spokesman said the 75 miles already built to replace weaker structures also helped tighten security.

“People can walk through or across a vehicle barrier with drugs or other contraband, but can’t walk through an 18- to 30-foot wall with anti-climb features, sensors and cameras,” the spokesman said.

Workers lay the foundation for new wall near the Rio Grande Valley river, an area with many crossings, just southeast of McAllen, Texas. (Courtesy of CBP)

The announcement comes as the Trump administration is making a renewed effort to show voters the U.S.-Mexico border wall remains a top priority.

Critics on the right have pushed back in recent months, saying border construction touted by the administration only represents replacement wall in areas that already had existing barriers and fencing.

Now, wall advocates are excited that the project is advancing and note that illegal crossings are falling dramatically from summer peaks, in part due to Mexico stepping up enforcement of its borders and in part thanks to renovated wall sections.

“They're making progress,” Mark Krikorian, the executive director of the Center for Immigration Studies, told Fox News. “They desperately need more barriers down there because that's one of the main places people are crossing.”

Customs and Border Patrol said that about 100 miles of wall are ultimately expected to go up in the Rio Grande Valley sector.

A new section of the border wall with Mexico in El Centro, Calif. (AP Photo/Jacquelyn Martin, File)

The new work is part of additional planned construction: a CBP document showed that contracts have already been awarded for the construction of 79 miles of new wall in areas where no fences exist currently. A CBP document showed maps of where the new wall is likely to be built.

By the end of 2020, the agency aims to have completed 141 miles of brand-new wall and 24 miles of new levee wall, along with 273 miles of replacement wall, for a total of more than 400 miles of border barriers. Seventy-one miles would also receive a second wall for extra security.

The funding for the project in Texas comes from congressional appropriations and is separate from construction funding authorized via an emergency order by President Trump. Border Protection estimated that each mile of wall costs about $17 million dollars.

Three migrants who managed to evade the Mexican National Guard and cross the Rio Grande onto U.S. territory walked along a border wall set back from the geographical border, in El Paso, Texas, as seen from Ciudad Juarez, Mexico. (AP Photo/Christian Chavez, File)

There are advocacy groups that oppose new wall construction.

Some of the opponents do so on environmental grounds, worrying that walls block animals from crossing the border, and that the barriers could trap animals between the Rio Grande and the wall in the event of a flood.

Others said a wall supports an intolerant immigration system.

“We adamantly reject this move by the Republican House leadership to hand over more than a billion dollars in taxpayer money for a monument to intolerance,” said UnidosUS, a Latino nonprofit advocacy group.

But Border Protection and others said a wall is crucial for giving America control over its borders.

In the fiscal year 2019, more than 800,000 people have been apprehended illegally crossing America’s southern border, CBP said.

Vehicle barriers that have already been replaced with a 30-foot wall along many miles of the border (Courtesy CBP)

Supporters of tighter border controls cheered the construction as the latest in a string of good news about strengthening border security.

“The apprehension numbers have been going down since the peak in May,” Krikorian said. Illegal crossings hit new records this past summer, but by September had fallen below 2016 levels for that month, according to CBP.

“Mexico is doing a better job of stopping people,” Krikorian said, also noting that the Trump administration has been closing the “catch and release” loophole by requiring asylum applicants to wait in Mexico while their cases are pending.

Mexico agreed to an arrangement to allow the U.S. to send foreign nationals to Mexico to wait after Trump said he would impose tariffs. Mexico also dramatically increased its own border protection, sending nearly 15,000 troops to its northern border to apprehend migrants before they crossed.

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Krikorian said that while he’d like for even more wall to have gone up, he understands that lawsuits and land use rights have slowed the progress.

Maxim Lott is executive producer of Stossel TV and creator of ElectionBettingOdds.com. He can be reached on Twitter at @MaximLott