The proposed wall along the US-Mexico border must be see-through to prevent people being hit on the head with sacks of drugs, Donald Trump has said.

The US President estimated "anywhere from 700 to 900 miles" of barricades were needed between the two countries, with mountains and rivers providing "natural barriers" along the rest of the 2,000-mile frontier.

A border wall to fight illegal immigration was one of Mr Trump's key election campaign promises.

The frontier, which stretches across four US states, already has 600 miles of fences and walls, although the President admits some some areas need replacement or repair.

Speaking on a flight from Washington to Paris, Mr Trump told reporters on board Air Force One that it was vital border agents could see through the wall to be aware of oncoming dangers.

"One of the things with the wall is you need transparency. You have to be able to see through it," he said.

"So it could be a steel wall with openings, but you have to have openings because you have to see what's on the other side of the wall.

"As horrible as it sounds, when they throw the large sacks of drugs over, and if you have people on the other side of the wall, you don't see them. They hit you on the head with 60 pounds of stuff? It's over."

The President added "violent and vicious" rivers provided a natural barrier along parts of the border.

The winding Rio Grande defines the frontier in most of Texas, though in parts it is little more than a trickle of water, while the Colorado River marks the boundary along 24 miles in Arizona.

Mr Trump said: "You have mountains. You have some rivers that are violent and vicious. You have some areas that are so far away that you don't really have people crossing.

"But you'll need anywhere from 700 to 900 miles."

The White House originally said Mr Trump's comments on Thursday were off the record, but later changed course and released his remarks.

Donald Trump says he still wants Mexico to pay for the wall at G20 summit

Andres Rozental, Mexico's former deputy foreign minister and ambassador to the UK, told Sky News the proposed see-through wall was "the most ridiculous thing I have heard in many, many years".

He added: "It just doesn't make any sense."

Mr Trump's vow that Mexico will pay for the border wall, which the Mexican government has insisted will not happen, has strained relations between the neighbouring nations.

He has since said he will find a way for Mexico to repay the US for construction of the wall but that Congress would need to fund it first.