A general view shows a newly built section of the U.S.-Mexico border fence at Sunland Park, U.S. opposite the Mexican border city of Ciudad Juarez, Mexico January 26, 2017. REUTERS/Jose Luis Gonzalez

WASHINGTON (Reuters) - The U.S. Department of Homeland Security has issued requests for proposals for prototypes for a wall along the Mexican border, saying ideally it should be 30 feet (9 meters) high and the wall facing the U.S. side should be “aesthetically pleasing in color.”

A wall to stem illegal immigration was one of Donald Trump’s main campaign promises and has been highly controversial. The president has vowed to make Mexico reimburse the United States for its cost but Mexico has repeatedly said it will not do so.

Earlier this week, the White House requested $3 billion more for Homeland Security, with some of that intended for planning and building the border wall.

According to one document posted online by U.S. Customs and Border Protection Friday night, the wall should be 30 feet high, built using concrete, and “physically imposing.” However, it says designs over 18 feet (5.5 meters) high could be acceptable.

“Designs with heights of less than 18 feet are not acceptable,” the document said. It said the wall should have features that do not allow people to climb over it and should prevent digging below the wall.

“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,” it said.

The other document requesting proposals has many of the same requirements but it does not specify that it be solid concrete.