PITTSBURGH • US President Donald Trump has made building a barrier to stop migrants on the border with Mexico a central plank of his presidency, but on Wednesday, he came up with a puzzling new twist to the initiative: a wall in Colorado.

The problem? Colorado lies well within the United States, though its large southern neighbour New Mexico does border Mexico.

"You know why we are going to win New Mexico? Because they want safety on their border," Mr Trump said during a speech in Pittsburgh.

"And we are building a wall on the border of New Mexico, and we are building a wall in Colorado," he continued. "We are building a beautiful wall, a big one that really works, that you can't get over, you can't get under."

"And we are building a wall in Texas, and we are not building a wall in Kansas, but they get the benefit of the walls that we just mentioned," he added.

Building a wall on the southern border with Mexico was one of the signature promises of Mr Trump's 2016 presidential campaign, but Congress has refused to pay for its construction.

Politicians and social media users reacted to Mr Trump's comment with bafflement and mockery.

"Well this is awkward... Colorado doesn't border Mexico," tweeted Mr Jared Polis, the state's Governor.

"Good thing Colorado now offers free full-day kindergarten so our kids can learn basic geography."

Democratic Senator Patrick Leahy tweeted a map showing a new international border drawn around New Mexico and a line obscuring the first half of the state's name so that it reads: Mexico.

Mr Trump later tweeted he had spoken of building a wall in Colorado "kiddingly", and "referred to people in the very packed auditorium, from Colorado & Kansas, getting the benefit of the Border Wall".

AGENCE FRANCE-PRESSE