US President Donald Trump has signed a directive to begin the construction of a wall on the border with Mexico, taking the first step to deliver on one of his most divisive campaign pledges.

“We've been talking about this right from the beginning,” the Republican president said as he signed the order during a ceremony for homeland security secretary on Wednesday, according to Reuters.

The wall will be stretched across the roughly 2,000-mile (3,200-kilometer) US-Mexico border.

Earlier, Trump said during an interview with ABC News that the construction was slated to begin within months, while planning would start immediately.

Trump also asserted that Mexico would reimburse “100 percent” of the wall’s costs.

The American president took another step aimed at curbing immigration by signing a separate executive action on public safety for the interior of the US, authorizing a crackdown on US cities that shield illegal immigrants.

The directive aims to strip federal funding from “sanctuary” states and cities that harbor illegal immigrants.

White House Press Secretary Sean Spicer said earlier in the day that the orders were the new administration’s “first step to really securing our porous border.”

“This will stem the flow of drugs, crime, illegal immigration into the United States,” he added.

During his campaign run, Trump repeatedly pledged to build a wall along the southern border to prevent more immigrants from entering the US illegally.

The Manhattan billionaire even estimated that erecting the wall would cost $8 billion, pledging to force Mexico to cover it.

Earlier this month, however, Trump joined other Republicans, saying US taxpayers may foot the initial bill for the proposed wall, insisting that Mexico would repay the US the money in the end.

Mexican President Enrique Pena Nieto made it clear during a meeting with Trump in September last year that his country would not pay for the project.

Mexican Foreign Minister Luis Videgaray repeated Nieto’s remarks on January 11, saying there was “no way” that his country would fund the wall.