Smuggling gangs in Mexico have repeatedly sawed through new sections of US President Donald Trump's border wall in recent months by using commercially available power tools, opening gaps large enough for people and drug loads to pass through, according to US agents and officials.

The breaches have been made using a popular cordless household tool known as a reciprocating saw that retails at hardware stores for as little as $US100 ($NZ155).

When fitted with specialised blades, the saws can slice through one of the barrier's steel-and-concrete bollards in a matter of minutes, according to the agents, who spoke on the condition of anonymity because they were not authorised to speak publicly about the barrier-defeating techniques.

After cutting through the base of a single bollard, smugglers can push the steel out of the way, allowing an adult to fit through the gap.

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Because the bollards are so tall - and are attached only to a panel at the very top - their length makes them easier to push aside once they have been cut and are left dangling, according to engineers consulted by The Washington Post.

The taxpayer-funded barrier - so far coming with a $US10 billion price tag - was a central theme of Trump's 2016 campaign, and he has made the project a physical symbol of his presidency, touting its construction progress in speeches, ads and tweets.

Manuel Balce Ceneta/AP President Donald Trump has boasted to crowds in recent weeks about the superlative properties of the new sections of border wall, calling them "virtually impenetrable".

Trump has increasingly boasted to crowds in recent weeks about the superlative properties of the barrier, calling it "virtually impenetrable" and likening the structure to a "Rolls-Royce" that border-crossers cannot get over, under or through.

The smuggling crews have been using other techniques, such as building makeshift ladders to scale and overtop the barriers, especially in the popular smuggling areas in and around San Diego, according to nearly a dozen US agents and current and former administration officials.

Mexican criminal organisations, which generate billions of dollars in smuggling profits, have enormous incentive to adapt their operations at the border to new obstacles and enforcement methods, officials say.

The US government has not disclosed the cutting incidents and breaches, and it is unclear how many times they have occurred. US Customs and Border Protection declined to provide information about the number of breaches, the location of the incidents and the process for repairing them.

One senior administration official, who was not authorised to discuss the breaches but spoke on the condition of anonymity, said they amounted to "a few instances" and that the new barrier fencing had "significantly increased security and deterrence" along sections of the border in CBP's San Diego and El Centro sectors in California.

Current and former CBP officials confirmed that there have been cutting breaches, but they said the new bollard system remains far superior and more formidable than any previous barrier design.

Some of the damage has happened in areas where construction crews have yet to complete the installation of electronic sensors that, once operational, will more quickly detect the vibrations sawing produces on the bollards, the officials said. They also said one of the main advantages of the steel bollard system - which stands between five and nine metres tall - is that damaged panels can be easily repaired or replaced.

Nick Miroff/WASHINGTON POST Workers install panels of steel bollard fencing along the US-Mexico border.

Ronald Vitiello, the former Border Patrol chief who was acting director of US Immigration and Customs Enforcement until his removal in April, characterised the breaches as "poking and prodding" by cartel smuggling crews.

"The cartels will continue to innovate, and they're not just going to leave San Diego because the wall gets better," Vitiello said. "That's life on the border."

Vitiello, who helped oversee the development of barrier prototypes in 2017, said the administration could have added better deterrent features if Democrats in Congress had provided more funding.

"The bollards are not the most evolved design; they are the most evolved that we could pay for," Vitiello said. "We never said they would be an end-all, be-all."

In the San Diego area, smugglers have figured out how to cut the bollards and return them to their original positions, disguising the breaches in the hope that they will go unnoticed and can be reused for repeated passage. Agents said they have learned to drive along the base of the structure looking for subtle defects, testing the metal by kicking the bollards with their boots.

If damage is detected, welding crews are promptly sent to make fixes. The smugglers, however, have returned to the same bollards and cut through the welds, agents say, because the metal is softer and the concrete at the core of the bollard has already been compromised. The smugglers have also tried to trick agents by applying a type of putty with a colour and texture that resembles a weld, making a severed bollard appear intact.

Agents in California and Texas said smuggling teams have also been using improvised ladders to go up and over the barriers, despite the risk of injury or death from falling; the tallest barriers are approximately the height of a three-storey building. Some of the teams deploy lightweight ladders made from metal rebar, using them to get past the "anti-climb panels" that span the top of the barrier.

Once the lead climber reaches the top, agents say, they use hooks to hang rope ladders down the other side.

Carolyn Van Houten/WASHINGTON POST Sections of fencing rest between the border barriers that separate the US and Mexico near San Diego.

The rebar ladders are popular because the metal support rods are inexpensive and are skinny enough to pass through the four-inch-wide gaps between the bollards, making it possible for the smuggling teams to use them to scale the secondary row of fencing, according to agents. Rebar, easily purchased at hardware stores, typically is used within concrete as reinforcement.

Trump initially wanted to build a concrete wall along the length of the border but was talked out of it by Homeland Security officials who said the bollard system is a superior design because it allows agents to see through to the other side.

"Frankly, an all-concrete wall would have been a much less expensive wall to build," Trump said in September during a visit to new sections of the barrier in San Diego. "But from the standpoint of Border Patrol, they were very much opposed to it."

CBP officials also have consistently said that no single structure, regardless of its design, can seal the border on its own. Rather, they have advocated for a "border wall system" that combines physical barriers, surveillance technology and the rapid deployment of agents to interdict border-crossers and attempted breaches.

"There's no one silver bullet, and we've done our best to try to explain that," said Chris Harris, a retired Border Patrol agent in San Diego. "You're always going to have to have boots on the ground. That's why there are armed police officers at Fort Knox."

Smugglers with reciprocating saws were able to cut through previous versions of the barrier in far less time, agents note, and the new bollard design makes the smugglers' work significantly more difficult.

Other Homeland Security officials note that the narrow gap created by a cut bollard permits only one person to pass through at a time, making it more difficult for large groups of migrants or smugglers to pass.

Because CBP is adding double-layer barriers in such high-traffic areas as San Diego, smugglers seek out locations where the distance between the primary and secondary fences is narrowest. A sawing crew will cut at a bollard while lookouts keep watch for US agents, so the smuggling team can run back into Mexico if US agents arrive. Once the agents leave, the smugglers can resume their sawing attempt in the same place.

"What happens any time some barrier is thrown up in front of a business is they adapt, and that's all they're trying to do," said Joshua Holmes, a Border Patrol agent and union official in San Diego.

The San Diego area is one of the most lucrative for narcotics smugglers or those bringing in migrants, who are willing to pay thousands of dollars apiece to reach the United States. Mexican deportees with homes and jobs on the US side of the border are among some of the best-paying customers because they often have assets and are desperate to return.

The Trump administration commissioned a set of border barrier prototypes in 2017, and among the tests CBP conducted were the structures' resilience against breaching with reciprocating saws, according to federal contracting documents and testing reports.

At the time, CBP agents said that no single design could be completely impenetrable, but the agency determined that steel bollards could not easily be cut without the use of "multiple power tools".

San Diego broadcaster KPBS, which reported on the prototype tests in 2018, obtained heavily redacted copies of the test results through the Freedom of Information Act. The reports showed that all of the designs the Trump administration evaluated in 2017 were found to be vulnerable to breaching methods.

NBC News subsequently published images of steel bollards that were cut during the prototype tests and showed the photographs to the president. "That's a wall designed by previous administrations," Trump said.

The version of the barrier being installed is based on the same bollard design, which the president calls "steel slats". It features square bollards with a steel exterior that and a core filled with commercial-grade concrete that is reinforced with metal rebar rods.

Kevin Trumble, a professor of materials engineering at Purdue University, and Srinivasan Chandrasekar, a professor of industrial engineering at Purdue, said a skilled operator with a reciprocating saw would be able to cut through the structure and that a severed bollard could be pushed out of the way using a standard car jack.

The engineers said other lithium battery tools could also be used to cut the steel and concrete. "You could use another device, like an abrasive saw, that would go even faster, but they create sparks because they operate at a high speed," Chandrasekar said.

The engineers estimated it would take someone 15 to 20 minutes to cut through a bollard or less if a team worked in pairs with two saws. The crews might need to go through multiple blades to complete a cut, the engineers said, but they can be quickly changed to resume sawing.

Online video demonstrations of reciprocating saws show that commercially available diamond grit and tungsten carbide blades are capable of slicing through thick pieces of steel and concrete in significantly less time.

The Trump administration has so far completed 122 kilometres of new barriers, all of it in areas like San Diego where the structure has replaced older, shorter and, in some cases, dilapidated fencing.

Another 254km of barrier is under construction, according to CBP, and the agency said 444km are in a "preconstruction" phase.

The administration remains on track to complete 724km of barriers by the end of next year, CBP acting commissioner Mark Morgan said last week.

The latest construction data obtained by The Washington Post show that the administration has finished just 2 per cent of the barrier planned for stretches of border in Texas, where plans call for 267km of new fencing. Almost all of that barrier would be built on private land that the government has yet to acquire.

Trump campaigned on a promise to make Mexico pay for construction of the barrier, but nearly $US10 billion that his administration has budgeted for the project has been taxpayer money from US funding sources, primarily the Defense Department.