US president Donald Trump has taken steps to make one of his biggest and most contentious campaign promises a reality by ordering the building of a wall along the country’s border with Mexico.

On his fifth full day in office, the Republican, who turned his signature policy to “build the wall” into one of the catchphrases of the 2016 election and a popular chant for his supporters, moved aggressively to strengthen US borders and immigration controls and crack down on criminals in the US illegally.

“A nation without borders is not a nation. Beginning today the United States of America gets back control of its borders,” Mr Trump declared on a visit to the Department of Homeland Security, the government agency that will put his plans into practice to cut off the flow of drugs and illegal immigration.

He signed two orders, including one that orders the “immediate” construction of “a contiguous, physical wall or similarly secure, contiguous and impassable physical barrier” along the 3,145km (1,954-mile) southern border.

The new president stuck with another campaign promise insisting that Mexico would pay for the multibillion wall, even though the Mexican government has repeatedly said they would not.

“I’m just telling you there will be a payment,” he told ABC News in his first interview at the White House. “It will be in a form, perhaps a complicated form.”

Construction would begin “in months”, he said.

Paying for the wall, for which estimates range from Mr Trump’s own figure $8 billion-$25 billion, would require the backing of the Republican-controlled Congress where some of his party oppose it,

The wall will likely be a crunch issue in the talks the Republican president has called for on the renegotiation of the North American Free Trade Agreement (Nafta) with Mexico and Canada.

Mexico’s economic minister Ildefonso Guajardo has warned that his country may withdraw from these renegotiations should the Trump administration demand payment for the wall.

‘Sanctuary’

Another of the other measures in Mr Trump’s presidential decrees that do not require the support of Congress is an order to strip government funds from so-called cities such as San Francisco and Chicago and states that offer “sanctuary” for undocumented immigrants by not enforcing federal immigration laws.

Mr Trump’s orders call for an increase in the country’s border patrol by an additional 5,000 agents and 10,000 new immigration and enforcement officers to carry out deportations.

White House press secretary Sean Spicer said that the focus of immigration actions would be illegal immigrants “who have also otherwise violated our laws”.

“After these criminals spend time in prison for the crimes they have committed, they are going to get back one-way tickets back to their country of origin,” he said.

Mr Spicer has said that Mr Trump’s priority on undocumented immigrants was to remove those individuals who have committed crimes in the US but he has refused to say whether other undocumented individuals would be deported under future plans.

Mr Trump said during the presidential campaign that he would deport all undocumented immigrants and that he would allow the “good ones” to return under an “expedited process.”

The focus appears to be on immigrants who have committed serious crimes.

“It is not as worrisome as it was in the first days after president Trump was elected,” said Chicago-based Senator Billy Lawless, who represents the Irish abroad in the Seanad.

“I think it is hardened criminals they are after. I am not sure they are looking for people who have run traffic lights or broken tail-lights or speeding.”

Mr Lawless said it was a “good omen” that Mr Trump’s orders did not include any crackdown on Mr Obama’s deferred action against young undocumented people brought to the US as children that protects some Irish families.

At the White House press briefing, Mr Spicer, whose great-grandfather emigrated from Kinsale in Co Cork, addressed an “issue that is near and dear to me”: he confirmed that Mr Trump had sent an invite to Taoiseach Enda Kenny to the White House on St Patrick’s Day and that they looked forward to his visit.