WASHINGTON—President Donald Trump, who is struggling to win congressional approval for his promised border wall with Mexico, now says that the project doesn’t have to stretch along the entire southern border.

“It’s a 2,000 mile border, but you don’t need 2,000 miles of wall because you have a lot of natural barriers,” he told reporters Wednesday on Air Force One en route to Paris. “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.”

Mr. Trump, who had previously suggested he wanted a wall that would run along the entire border, said the wall could be more modest. “You’ll need anywhere from 700 to 900 miles,” he said.

It wasn’t clear whether that estimate included the 654 miles of border that already have some sort of fencing on it. A White House spokesman didn’t immediately respond to a request seeking clarification.

The Customs and Border Protection Agency has said that about 130 miles of the border aren’t appropriate for a wall due to lakes, mountains and other natural barriers.