Washington (AFP) - US President Donald Trump pitched a new concept to his supporters for the wall he intends to erect on the Mexican border: cover it with solar panels -- and use the energy to cover construction costs.

"Yes, we will build a wall," he told a crowd in Cedar Rapids, Iowa. "We have to stop the drugs from flowing in."

"I will give you an idea that nobody has heard about yet," he said. "The southern border. Lots of sun, lots of heat. We are thinking about building a wall as a solar wall. So it creates energy. And pays for itself."

"And this way, Mexico will have to pay much less money. And that's good. Right?" quipped the president, whose initial pledge to make Mexico pay for the wall has met with stiff resistance from America's southern neighbor.

"Think of it, the higher it goes, the more valuable it is," he enthused.

"Pretty good imagination, right? My idea!"

The US administration put out a call for proposals several months ago for the construction of the border wall, one of which -- submitted by a Las Vegas businessman named Tom Gleason -- involved using solar panels.

The Trump administration has yet to make serious headway on the president's emblematic but hugely costly campaign pledge to build a wall on the Mexican border.

Under pressure from Democrats, the US Congress has so far refused to commit funding to the project, agreeing only to finance maintenance on existing parts of the border fence.

The real funding battle will play out starting in October, when 2018 budget negotiations begin in earnest.