Eight prototypes for US President Donald Trump's proposed Mexico border wall are taking shape at a construction site in San Diego.

The prototypes, which cost the US Government up to $US500,000 each, form a tightly packed row of imposing concrete and metal panels.

One has sharp metal edges on top, while another has a surface resembling an expensive brick driveway.

The models, spaced around 9 metres apart, stand in the dirt just a few steps from homes in Tijuana, Mexico.

As crews worked on one model recently, three men and two women from Nepal jumped a short rusted fence from Tijuana into the construction site and were immediately stopped by Border Patrol agents on horseback.

Border Patrol spokesman Theron Francisco said there had been four or five other illegal crossing attempts at the site since work began on September 26.

Slopes, thickness and curves vary. One has two shades of blue with white trim. The others are grey, tan or brown — in sync with the desert.

Bidding guidelines call for the prototypes to stand between 5.5 and 9.1 metres high and be able to withstand at least an hour of punishment from a sledgehammer, pickaxe, torch, chisel or battery-operated tools.

Features also should prevent the use of climbing aids such as grappling hooks, and the segments must be "aesthetically pleasing" when viewed from the US side.

The administration hasn't said how many winners it will pick or whether Mr Trump will weigh in himself.

There is currently 1,052 kilometres of single-layer fence on the 3,143-kilometre border, plus 82 kilometres of double- and triple-layer fence.

"I'm sure they will engage in a lot of tests against these structures to see how they function with different challenges," US representative Bob Goodlatte, a Virginia Republican who chairs the House Judiciary Committee, said after touring the construction site.

Mr Trump has asked Congress for $US1.6 billion to replace 22.4 kilometres of wall in San Diego and build 96 kilometres in Texas' Rio Grande Valley, the busiest corridor for illegal crossings.

AP