Migrants and smugglers have succeeded in cutting through US President Donald Trump's "virtually inpenetrable" border wall using cordless saws that cost as little as $US100 ($A145) apiece at hardware stores, border agents and officials told The Washington Post.

Trump administration officials admitted being aware of "a few instances" in which human smugglers managed to breach rebuilt sections of the $US10-billion wall that Trump once promised Mexico would pay for, the paper reported.

The widely available saws can cut through the wall's steel bollards within a matter of minutes. The nine metre panels can then be easily pushed forward, allowing people to pass through.

Most of the reported border crossings involving cuts through the wall have occurred in the busy area near San Diego [in southern California], the paper said.

Past and present border officials insisted the newly reinforced sections of wall are still harder to cut through than they were before. They attributed the new breaches to the constant innovation of smuggling gangs, which amount to a multibillion-dollar industry of sneaking undocumented immigrants into the US.

"They're not just going to leave San Diego because the wall gets better," Ronald Vitiello, a former US Border Patrol and US Immigration and Customs Enforcement chief, told the Post. "That's life on the border."

Trump touts the wall as almost impossible to get through and has brought reporters along to show off what he calls the "Rolls Royce" of walls.

He has diverted billions in taxpayer funds that Congress approved for other uses to rebuild sections of the wall after famously vowing that Mexico would pay for it.

Australian Associated Press