Plans for a border wall dividing America from Mexico took a small step forward as the government announced who had won contracts to build prototypes for the barrier.

While a Customs and Border Patrol official portrayed the news as a significant step for Donald Trump’s signature campaign promise, the wall Mr Trump promised his supporters has faced legal challenges and ongoing questions about Congress’ willingness to allocate the billions needed for construction. An internal Department of Homeland Security study that leaked to the media estimated the cost at $21.6 billion.

That total dwarfs the cost of awarding contracts to four companies chosen to build concrete models at a cost of around $400,000 to $500,000 each according to United States Customs and Border Patrol said. A call for proposals stipulated structures that are 30 feet tall, deter climbing and are see-through - as the president demanded.

In a conference announcing the contracts, Deputy Commissioner Ronald Vitiello displayed images of an 150-foot “enforcement zone” that border agents would be able to monitor.

“We’re not just asking for a physical structure. We’re asking for all the tools that help us secure the border”, Mr Vitiello said.

“There’s nothing lethal in this”, Mr Vitiello added in response to a question about features allowing agents to halt would-be border crossers.

Life on the US-Mexican border Show all 12 1 /12 Life on the US-Mexican border Life on the US-Mexican border Tijuana, Mexico A house stands next to a section of the border fence separating Mexico and the US Reuters Life on the US-Mexican border Tijuana, Mexico The border fence in Tijuana stretches all the way to beach Reuters Life on the US-Mexican border Tijuana, Mexico Migrants can often be spotted trying to cross the rusty barrier Reuters Life on the US-Mexican border Tijuana, Mexico Joaquin, 36, a chef from Guatemala who says he was deported from the US Reuters Life on the US-Mexican border Tijuana, Mexico A shack stands next to a section of the border fence Reuters Life on the US-Mexican border Tijuana, Mexico Tourists pose for a picture at the border Reuters Life on the US-Mexican border Tijuana, Mexico A family burns rubbish near the fence Reuters Life on the US-Mexican border Tijuana, Mexico Joaquin makes a living by selling rubbish Reuters Life on the US-Mexican border Tijuana, Mexico ‘Neither Trump nor the wall is going to stop anyone,’ says Pedro Reuters Life on the US-Mexican border Tijuana, Mexico Mexicans have built homes right next to the fence Reuters Life on the US-Mexican border Tijuana, Mexico Mexican Carlos, 27, who says that he was deported from the United States, heats up tortillas at his house near the double fence that separates Mexico and the US Reuters Life on the US-Mexican border Tijuana, Mexico A girl climbs stairs near a section of the fence

The announcement came as Donald Trump has struggled to finance his central campaign promise. Mexico has flatly rejected paying for it, despite Mr Trump assuring his supporters to the contrary, and Mr Trump has threatened a government shutdown if Congress does not allocate needed funding.

Despite those headwinds, Mr Vitiello portrayed the contract awards as a “significant milestone”.