President Donald Trump has started the clock ticking on his pledge to build a wall between the United States and Mexico, and believes construction will start 'in months.'

He spoke in his first interview since being inaugurated, with ABC Nightly News anchor David Muir.

ABC News broke into lunch-hour programming – Washington, D.C. viewers missed a few minutes of 'Who Wants to be a Millionaire?' – in order to broadcast a brief teaser from Wednesday's Oval Office interview.

Construction will begin 'as soon as we can, as soon as we can physically do it,' Trump said.

Asked if it would be just a matter of months, the president agreed.

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BUILD THAT WALL: Asked for a timescale for construction he said: 'I would say in months, yeah. I would say in months. Certainly planning is starting immediately.'

BORDER BARRIER: A section of the border is already fenced at El Paso. Other section also have a high fence but Trump has promised a wall

DEFINITIVE: Trump repeated his frequent campaign claim that Mexico will, one way or another, 'absolutely, 100 per cent' pay for the wall.

'I would say in months, yeah. I would say in months. Certainly planning is starting immediately.'

Trump repeated his frequent campaign claim that Mexico will, one way or another, 'absolutely, 100 per cent' pay for the wall.

The president believes the giant public works project will dramatically reduce the northward flow of illegal immigrants and narcotics into U.S. border states and beyond.

On Wednesday he provided one of his vaguest explanations yet of how that payment would be structured.

'Ultimately it'll come out of what's happening with Mexico. We're going to be starting those negotiations relatively soon, and we will be, in a form. reimbursed by Mexico,' Trump said.

'It will be in a form, perhaps a complicated form,' he added, putting less meat on his idea's bones than usual.

Trump dismissed Mexican president Enrique Peña Nieto's steadfast denials that his country would ever pay to erect a wall on its northern edge.

Peña Nieto told his nation's diplomatic corps two weeks ago that he has 'differences with the new United States government on some issues, such as a wall that Mexico absolutely will not pay for. At no time will we accept anything that goes against our dignity as a country and our dignity as Mexicans.'

Trump scoffed.

'Well, I think he has to say that. He has to say that,' he declared. 'But I'm just telling you there will be a payment.'

FIRST INTERVIEW: Trump tells David Muir that the wall's go-ahead is 'good for the United States'.'It's also going to be good for Mexico,' he said.

NO: Mexican president Enrique Peña Nieto told his nation's diplomatic corps two weeks ago of the wall that 'Mexico absolutely will not pay for' it

'What I'm doing is good for the United States,' the president declared. 'It's also going to be good for Mexico. We want to have a very stable, very solid Mexico.'

ABC's David Muir and Jonathan Karl interviewed Trump at the White House, and described an exchange about the president's views on torturing suspected combatants in the War on Terror.

'This is a fascinating conversation,' Karl recalled, describing differences of opinion between Trump and Secretary of Defense James Mattis.

'He tells you that General Mattis surprised him by saying he doesn't think that waterboarding works, he doesn't think that torture works,' Karl said.

'And then he made it very clear that Donald Trump still very much believes that torture works, and he said in the last 24 hours he has spoken to top intelligence people who say it does work.'

'So for now,' the newsman concluded, 'no waterboarding, no torture. But he made it clear that he could bring it back.'

'Beginning today the United States of America ...gets back its borders': Trump signs orders to build Mexican wall and attack sanctuary cities, announces 5,000 new border guards and triples immigration enforcement force as he goes to war on immigration

President Donald Trump cracked down on illegal immigration on Wednesday, signing off on two executive actions to restore law and order.

One of the orders he signed jumpstarts construction of a wall on the border with Mexico, the White House says, the other defunds sanctuary cities like Los Angeles, New York and Chicago that harbor illegal immigrants.

'A nation without borders is not a nation,' Trump told Department of Homeland Security employees after signing the orders. 'Beginning today the United States of America...gets back its borders.'

To help with the 'crisis' at the southern border, Trump gave DHS the authority to hire another 5,000 border patrol officers and triple its roster of immigration enforcement agents.

President Donald Trump is cracking down on illegal immigration, issuing two executive actions this afternoon that aim to restore law and order

For 'too long' immigration official and border patrol agents 'haven't been allowed to properly do their jobs.

'That's all about to change,' Trump told employees of DHS. 'And I'm very happy about it, and you're happy about it.' Immigration laws will be 'enforced and enforced strongly,' he added.

The president said he was recognizing members of those agencies in his remarks 'not because they unanimously endorsed me for president.

'That helps, but that's not the only reason,' he teased.

Trump signed the executive orders this afternoon at DHS before his address.

He said in the remarks that followed that he expects the United States' relationship with Mexico to improve as a result of his actions because the 'unprecedented surge' of illegal immigrants in the United States is bad for both countries.

'It's going to be very, very good for Mexico,' he said of the orders' effects.

Trump said he's looking forward to meeting with Mexican President Enrique Pena Nieto next week for the first time as president, and the second time since the start of his campaign, to discuss the measures.

'I want to emphasize that we will be working in particular with our friends in Mexico to improve safety and economic opportunity on both sides of the border,' Trump said Wednesday. 'I have deep admiration [for] the people of Mexico.'

A proposal under consideration earlier this month had the U.S. footing the bill for the border wall 'for the sake of speed,' Trump said, and Mexico paying the money back.

He told ABC News in an interview that will air in full tonight, 'We're going to be starting those negotiations relatively soon. And we will be - in a form - reimbursed by Mexico.'

'I'm just telling you that there will be a payment. It will be in a form - perhaps a complicated form,' he said. 'And you have to understand. What I'm doing is good for the United States.'

At his daily briefing, Trump's spokesman previewed the executive orders Trump would be signing and what they entailed.

The first one called for 'Border Security and Immigration Enforcement Improvements.'

'It addresses long overdue border security issues and is the first order in that to build a large physical barrier on the southern border,' Spicer said. 'Building this barrier is more than just a campaign promise it's a common sense first step to really securing our porous border.

Adding, 'And yes, one way or another, as the president said before, Mexico will pay for it.'

'We are going to get the bad ones out,' Trump told Department of Homeland Security employees after signing the orders, zeroing on criminals and drug dealers. 'The day is over where they can stay in our country and wreak havoc. We are going to get them out and get them out fast'

The order will empower immigration officials to do their jobs, he said, and create more detention spacer at facilities along the border.

'Under the Constitution, the American people get the final say on who can and cannot enter our nation and they've spoken loud and clearly through our laws.'

Trump will also end the previous administration's 'catch and release policy,' Spicer said, 'which has led to the deaths of many Americans.'

The president's directive continues one Obama-era policy. It places a priority on the 'the prosecution and deportation of illegal immigrants who've also otherwise violated our laws,' the White House said.

'And after these criminals spend time in prison for the crimes they've committed they're going to get back one-way tickets to the country of their origin. And their governments are going to take them back,' Spicer declared.

Trump said later at DHS: 'We are going to get the bad ones out.'

Zeroing on criminals and drug dealers, he said, 'The day is over where they can stay in our country and wreak havoc. We are going to get them out and get them out fast.'

The second document deals with sanctuary cities. Trump had said he would with Congress to cut off their access to federal taxpayer dollars until they obey national immigration laws.

'Federal agents are going to unapologetically enforce the law, no ifs ands or butts,' Spicer said today.

The order Trump signed today would affect major U.S. cities including Boston, New York, Los Angeles, Chicago, San Francisco and Philadelphia.

Chicago Mayor Rahm Emanuel came to visit Trump in New York after the election to ask him not to move forward on the campaign promise. Emanuel has said Chicago will remain a sanctuary city regardless of federal action like the one Trump took today

The White House said those cities would still have access to funding for policing.

'There's other aid that can extended as well, either through the U.S. attorneys office or other means that will ensure that the people of Chicago have the resources to feel safe,' Spicer stated.

Chicago Mayor Rahm Emanuel came to visit Trump in New York after the election to ask him not to move forward on the campaign promise. Emanuel has said Chicago will remain a sanctuary city regardless of federal action like the one Trump took today.

'We'll hopefully get a dialogue started with Mayor Emanuel and try and figure out what a path forward can be, so we get - so we come up with a plan that can keep the people of Chicago safe and help ease the problem there,' Spicer said.

Trump's spokesman also hinted at another executive order dealing with immigration that's coming down the hatch: 'extreme vetting.'

Spicer suggested Wednesday that the administration has an order in the works to limit immigration from countries afflicted by terrorism.

'You’ll see more action this week on keeping America safe,' he said. 'This has been something he talked about in the inaugural address. He talked about it in the campaign. As we get into the implementation of that executive order, we’ll have further details.'

Trump was at DHS to deliver remarks at attend the ceremonial swearing in of his Homeland Security chief, John Kelly, who took his formal oath of office last Friday.

'He's a rough tough guy but he's also got a good heart,' Trump said and 'will do a very special job.'