US President Donald Trump has signed two executive orders - one on border security and the intent to build a wall along the border with Mexico, and another strengthening the enforcement of immigration laws.
"Building this barrier is more than just a campaign promise. It's a common sense first step to really securing our porous border," said White House spokesman Sean Spicer on Thursday (NZ time).
"This will stem the flow of drugs, crime, illegal immigration into the United States."
He maintained Mr Trump's insistence Mexico would pay for the wall "one way or another". Mexico has been ardent in its refusal to pay.
Construction could start within months.
Speaking to a crowd at the Department of Homeland Security following the signing, Mr Trump says the US was "in the middle of a crisis on our southern border".
"The unprecedented surge of illegal migrants from Central America is harming both Mexico and the United States and I believe the steps we will take starting right now, will improve the safety of both of our countries.
"A nation without borders is not a nation. Beginning today, the United States of America gets back control of its borders, gets back its borders," he said as the crowd of uniformed staff applauded.
The decree comes on the same day as Mexico's Foreign Minister Luis Videgaray was set to arrive in Washington for talks with the US government, while Mexican President Enrique Peña Nieto is expected to meet Mr Trump next week.
Thursday's announcement is part of a crackdown on immigration, which will also include detention centres along the southern border to "make it cheaper and easier to return them to their country of origin".
He said the orders will also strip federal money from so-called sanctuary cities and end a "catch and release" policy of previous administrations.
That policy saw immigrants caught illegally in the US released while they await a hearing with an immigration judge.
Mr Spicer claimed it had "led to the deaths of many Americans".
The Trump administration will start prioritising the prosecution and deportation of illegal immigrants who have also broken other US laws.
"And after these criminals spend time in jail for the crimes they've committed, they're going to get back one-way tickets to their country of origin and their governments are going to take them back," Mr Spicer said.
A programme which would see officials "target illegal immigrants for removal" would also be revived.
Reuters / Newshub.