US President Donald Trump took a first step toward fulfilling his pledge to “build a wall” on the Mexican border on Wednesday, signing two immigration-related decrees.

He visited the Department of Homeland Security to approve an order to begin work to “build a large physical barrier on the southern border,” according to the White House.

President Trump also signed measures to “create more detention space for illegal immigrants along the southern border” according to White House spokesperson Sean Spicer.

“We’re going to once again prioritise the prosecution and deportation of illegal immigrants who have also otherwise violated our laws,” he added.

Some of the border is already fenced, but Trump says a wall is needed to stop illegal immigrants entering from Latin America.

In 2014, there were an estimated 5.8 million unauthorised Mexican migrants in the United States, according to Pew, with fewer arriving each year before that.

Experts have voiced doubts about whether a wall would actually stem illegal immigration, or if it is worth the billions it is expected to cost.

But the policy has become a clarion call for the US right and far-right – the core of Trump’s support.

Still, any action from the White House would be piecemeal, diverting only existing funds toward the project