U.S. President Donald Trump stood by unverified claims of massive voter fraud, trumpeted his "home run" speech at the CIA, insisted Mexico will pay for a border wall and said waterboarding "absolutely works" as a method of interrogation, in his first one-on-one television interview since assuming the presidency.

Speaking to ABC's David Muir, Trump touched on many of the issues he's dealt with since taking office last Friday.

Here are some of the major points he addressed:

On ISIS and torture

Trump said the United States needs to "fight fire with fire" in combatting the Islamic State militants he says are "doing things that nobody has ever heard of since medieval times."

"They're chopping off the heads of our people and other people," Trump said of Islamic State. "So we have that and we're not allowed to do anything. We're not on an even playing field."

Trump said he plans to heed the advice of Secretary of Defence James Mattis and new CIA Director Mike Pompeo, when it comes to employing waterboarding as an interrogation method. The president said he's spoken to many intelligence officials about the effectiveness of waterboarding, and came away convinced.

"I want to do everything within the bounds of what you're allowed to do legally," Trump said. "But do I feel it works? Absolutely I feel it works."

Later in the interview, Trump said Islamic State would not exist if the U.S. had seized oil wells for itself during military conflict in the Middle East.

"Had we taken the oil, you wouldn't have ISIS because they fuel themselves with the oil," he said. He also condemned the U.S. withdrawal from Iraq, saying: "We created a vacuum and ISIS formed, but had we taken the oil something else would've very good happened."

Muir urged Trump to elaborate on previous comments, in which he suggest the U.S. might "get another chance" to take oil from the Middle East.

"Don't let it get your attention too much because we'll see what happens," he said.

On immigration restrictions from Muslim countries

Trump insisted that an upcoming executive order to tighten immigration restrictions will not be a "Muslim ban," but rather a restriction on "countries that have tremendous terror."

"Our country has enough problems without allowing people to come in who, in many cases or in some cases, are looking to do tremendous destruction."

Trump is expected to suspend issuing visas to citizens of several predominantly Muslim countries, including Syria, Iraq, Iran, Libya, Somalia, Sudan and Yemen.

Trump did not confirm that list of countries, but he acknowledged that Afghanistan, Pakistan and Saudi Arabia will not be on the list.

Nevertheless, he said there will be "extreme vetting" in place for everyone entering the United States.

The president said he is not concerned that his actions could anger Muslims around the world.

"There's plenty of anger right now. How can you have more?" he said.

He also said the U.S. would support "safe zones" for Syrian refugees fleeing conflict overseas.

Paying for the wall with Mexico

Trump was steadfast in declaring that Mexico will pay back the United States for what will be a taxpayer-funded wall along the border between the two countries. However, he left some wiggle room in his wording, saying that Mexico will reimburse the U.S. "in a form."

"It will be in a form, perhaps a complicated form," he said.

Muir pressed Trump, saying: "So they'll pay us back?"

"Yeah, absolutely, 100 per cent," he said.

Trump said the U.S. will be reimbursed "at a later date," and insisted he would not hold off on construction to secure the reimbursement deal. "We have to stop the drugs from pouring in, we have to stop people from just pouring into our country," he said. "But that wall will cost us nothing."

On Wednesday, Mexican President Enrique Pena Nieto said that his country will not foot the bill for Trump's campaign promise. "Mexico does not believe in walls," Nieto said. "Mexico will not pay for any wall."

Trump threatened to cancel a meeting with Nieto to discuss trade on Thursday, in response to the Mexican president's remarks. "If Mexico is unwilling to pay for the badly needed wall, then it would be better to cancel the upcoming meeting," he .

Nieto ultimately cancelled the meeting himself.

Trump said his administration will reveal its policy to address illegal immigrants already in the U.S., "over the next period of four weeks."

Losing the popular vote

Trump blamed Democrats in Congress for leaking his unverified claims that he lost the popular vote due to 3-5 million illegal votes, saying his comments were misrepresented after a "confidential meeting" with congressional leaders.

He did not respond to questions about his tweets alleging voter fraud prior to the meeting with congressional leaders.

"I would've won the popular vote if I was campaigning for the popular vote," Trump said. "I would've gone to California where I didn't go at all. I would've gone to New York where I didn't campaign at all."

Trump has said he will launch an investigation into voter fraud in the election. "There are millions of votes, in my opinion," Trump said. "You have people that are registered who are dead, who are illegals, who are in two states. You have people registered in two states."

Muir confronted Trump about the sources used to prove his claims, saying they've been "debunked."

"No it hasn't, take a look at the ," Trump said.

"I called the author of the Pew report last night, and he told me that they found no evidence of voter fraud," Muir said.

"Then why did he write the report?" Trump said. "He's groveling again."

Trump claimed that "none" of the alleged illegal votes were cast for him.

"I had a tremendous victory, one of the greatest victories ever," he said.

On being presidential

"I don't want to change too much. I can be the most presidential person ever, other than possibly the great Abe Lincoln, alright. I can be the most presidential person, but I may not be able to do the job nearly as well if I do that."

On his 'home run' speech at the CIA and inauguration crowd

Muir asked Trump about the number-related battles fought over his election results, TV ratings and inauguration crowd size, asking: "When does all of that matter just a little less? When do you let it roll off your back now that you're president?"

Trump responded with a lengthy discussion of his speech at the CIA on Saturday, calling it a "home run" that most networks allegedly tried to "downplay," except for Fox News.

"I got a standing ovation. In fact, it was the biggest standing ovation since Peyton Manning had won the Super Bowl and (Fox News) said it was equal. I got a standing ovation. It lasted for a long period of time."

Trump continued: "People loved it. They loved it. They gave me a standing ovation for a long period of time. They never even sat down, most of them, during the speech. There was love in the room."

He continued: "That speech was a big hit, a big success. Success."

Trump's scripted speech at the CIA focused largely on mending fences with the intelligence agency, but he later ventured into unscripted territory with a about "dishonest" journalists whom he claimed lied about the crowds at his inauguration.

He also boasted about the size of the crowd at his inauguration, saying: "We had the biggest audience in the history of inaugural speeches."

On 'sending in the feds' to help Chicago

Trump compared violence in Chicago to Afghanistan, in response to a question about his Twitter threat to "send in the feds" if Chicago doesn't "fix the horrible 'carnage.'"

"If they want help, I would love to help them," he said. "I will send in what we have to send in. Maybe they're not going to have to be so politically correct. Maybe they're being overly politically correct."

Trump said officials in Chicago are "going to have to get tougher and stronger and smarter. But they've got to fix the problem."

On receiving the nuclear codes

Trump said he recognizes the "scary" power of possessing the codes to launch the U.S.nuclear arsenal, but it doesn't keep him up at night.

"I have confidence that I'll do the right thing, the right job," he said. "When they explain what it represents and the kind of destruction that you're talking about, it is a very sobering moment. Yes, it's very very very scary in a sense."

On Obamacare

Trump repeated many of his criticisms of the Affordable Care Act passed under former president Barack Obama's administration. "It's too expensive. It's horrible healthcare. It doesn't cover what you have to cover," Trump said.

Trump failed to offer any specifics on his replacement plan, saying instead that he will "unleash something that's going to be terrific," so that "nobody's going to be dying on the streets." He also suggested the best course of action for the U.S. would be to allow "Obamacare" to "explode."

"But I don't want to do that," he said. "I want to give great healthcare at a much lower cost."

When asked if Americans who like their current health insurance coverage will lose it, Trump said: "We want the answer to be no one."