Requirements for the US-Mexico border wall released by the US federal government's Customs and Border Protection office late on Friday state that the wall must look appealing - but only on the American side.

In the notice for potential contractors, Customs and Border Protection (CBP) state that: "The wall design shall be physically imposing in height," with requirements of at least 30 ft although proposals of 18 ft will also be considered.

Building on President Trump's promise to make a "big, beautiful wall", the notice says that the wall must be aesthetically pleasing in colour and its anti-climb texture, and that it should blend in and be consistent with the general surrounding environment.

The notice only mentions however, aesthetic requirements for the north side of the wall, "i.e. U.S. facing side", with no address to having pleasing aesthetics for the Mexican side.

The wall must also not be possible for a human to climb to the top of the wall or access the top of the wall from either side unassisted, such as by using a ladder, and the wall design "wall design shall include anti-climb topping features that prevent scaling using common and more sophisticated climbing aids (e.g. grappling hooks, handholds, etc.)"

It must withstand attempts to cut through it - or under it

The notice specifically mentions that the wall must successfully endure for at least 30 minutes, but ideally more than four hours, attempts to cut through it with a "sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools".

The wall should also be designed to prevent digging or tunneling below it for a minimum of 6 feet.

Other proposals in the notice require that contractors buy "American-Construction Materials", with an exception to buy foreign materials if the government determines that the cost of domestic construction material would be unreasonable.

In the next two weeks, the CBP will select from admitted proposals several 30-foot-long prototypes to be built.

Each prototype is expected to cost between $200,000 and $500,000, according to the notice.

Although Friday's release did not mention the overall cost of the wall, various estimates have put the project at between $10 billion and $25 billion.

Trump's recent budget proposal requested $1 billion to begin the process.