President Donald Trump has said he was "not joking" about his proposed plan to build a solar- powered wall on the US-Mexico border with a see-through provision for security reasons. "No, not joking, no. There is a chance that we can do a solar wall," Trump told reporters aboard Air Force One travelling with him to Paris from Washington. Major companies are looking at that, he said. Trump said that "there is a very good chance" a solar wall could be built, and that "there is no better place for solar than the Mexico border \u2014 the southern border." He said the wall would have to be see-through, a demand that is been made by some Border Patrol officials who oversee the border. "You have to be able to see through it. In other words, if you can't see through that wall - so it could be a steel wall with openings, but you have to have openings because you have to see what is on the other side of the wall," Trump added. "As horrible as it sounds, when they throw 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?\u00a0 It's over. As crazy as that sounds, you need transparency through that wall. But we have some incredible designs," he said. You may also want to watch: Trump also admitted that his proposed wall would not cover the entire 2,000-mile border. "And remember this, it is a 2,000 mile border, but you don't need 2,000 miles of wall because you have a lot of natural barriers," he said. Trump said there was a need for 700-900 miles of border wall. There are currently about 600 miles of different types of fencing on the US-Mexico border. "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. So you don't need that. But you'll need anywhere from 700 to 900 miles," Trump said. Democrats have threatened to derail any budget proposal that includes specific funding for construction of new stretches of the wall. But Trump said the existing barriers are part of the wall that he made a central promise of his campaign. "Plus we have some wall that's already up that we're already fixing. You know, we've already started the wall because we're fixing large portions of wall right now. We're taking wall that was good but it's in very bad shape, and we're making it new. We're fixing it. It's already started," Trump said.