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President Donald Trump is expected to sign an executive order Wednesday to begin paying for a wall on the U.S.-Mexican border, a senior administration official told NBC News on Tuesday night, taking the first step toward fulfilling his marquee campaign promise.

The official, who asked not to be identified, said Trump would likely sign the order — which would shift money from other federal programs to the wall project — during an appearance Wednesday at the Department of Homeland Security, the parent agency of U.S. Customs and Border Protection.

The official said Trump still intends for Mexico to end up paying for the wall eventually.

The president signaled the move on Twitter on Tuesday night.

Big day planned on NATIONAL SECURITY tomorrow. Among many other things, we will build the wall! — Donald J. Trump (@realDonaldTrump) January 25, 2017

The specific effect of the order remains unclear, as Congress would have to approve any extra appropriations.

Trump's promise to build a wall and to "make Mexico pay for it" were keynotes of his presidential campaign. He said in an interview on CBS's "60 Minutes" on Nov. 13, five days after he was elected, that he would accept building a fence instead of a wall "in certain areas."

The senior administration official said many more immigration-related executive orders are expected this week.