The Department of Homeland Security has asked for Mexico border wall designs that are "aesthetically pleasing" to the US side of the fence.
The prototype proposals requested by the department were asked to be at least 9-metres (30 ft) high and should feature details that prevent people from digging underneath it, Reuters reported.
A document released online by U.S. Customs and Border Protection said the wall should be "physically imposing" but be "aesthetically pleasing in colour".
It added: "The wall shall prevent/deter for a minimum of 1 hour the creation of a physical breach of the wall (e.g., punching through the wall) larger than 12 inches (30 cm) in diameter or square using sledgehammer, car jack, pickaxe, chisel, battery-operated impact tools, battery-operated cutting tools, oxy/acetylene torch or other similar hand-held tools."
The current fencing along parts of the US/Mexico border is frequently subject to cuts made using various tools – although it is not yet clear what designs would prevent similar attempts to punch through the wall.
The topic has caused something of a rift between US President Donald Trump and Mexican President Enrique Pena Nieto.
Trump, who pledged to build a wall on the US southern border during his presidential campaign, has long insisted Mexico should pay for the wall, while Pena Nieto has made it clear his country will not be footing the bill.
The US administration has come up with a variety of suggestions for funding, including putting a high tax on Mexican imports and using US taxpayers money upfront.