Donald Trump has signed executive order directing the Department of Homeland Security that calls for a "large physical barrier" between the US and Mexico. 

One of two actions, the wall was one of Mr Trump's primary campaign promises, and his order will jump-start the process for constructing the barrier. In addition to the wall, Mr Trump signed an action to strip federal funding from "sanctuary cities", that do not prosecute undocumented immigrants. 

The President signed the orders during a ceremony at DHS headquarters, honouring the newly confirmed Gen John Kelly as director.

"As soon as we can, as soon as we can physically do it,"  he told ABC News. "I would say in months, yeah. I would say in months – certainly planning is starting immediately."

Mr Trump added that US taxpayers will fund the wall, but claimed that they will be "reimbursed" by Mexico. He did not explain how. 

"All it is, is we'll be reimbursed at a later date from whatever transaction we make from Mexico," he said.

"I'm just telling you there will be a payment. It will be in a form, perhaps a complicated form. What I'm doing is good for the United States. It's also going to be good for Mexico. We want to have a very stable, very solid Mexico."

Press Secretary Sean Spicer told reporters that Mexico will pay "one way or another".