In his first one-to-one television interview since his inauguration, the US President said senior officials had told him that so-called enhanced interrogation techniques did work.
His administration is reportedly seeking a review of how the US is conducting its war on terror, including interrogation techniques such as waterboarding and whether CIA “black site” prisons overseas should be reopened.
Torture techniques are regarded as illegal under US and international law.
Mr Trump said he would be guided by his cabinet members – including CIA director Mike Pompeo, who has ruled out using torture, and defence secretary James Mattis, who told the President that it is counter-productive.
But the President told ABC News: “I’m going to go with what they say and if they don’t want to do, that’s fine.
“If they do want to do, then I will work toward that end.
“I want to do everything within the bounds of what you’re allowed to do legally.”
It reveals once again Mr Trump’s willingness to repeat in the Oval Office some of his incendiary language from the campaign trail.
He said his aim was to keep Americans safe and defeat terror.
“When ISIS is doing things that no one has ever heard of, since medieval times, would I feel strongly about waterboarding?
“As far as I’m concerned, we have to fight fire with fire.
“We’re not playing on an even field.
“I want to do everything within the bounds of what you’re allowed to do legally.
“But do I feel it works? Absolutely, I feel it works.”
His words drew criticism from former Republican presidential candidate John McCain. “We are not bringing back torture in the United States,” he said.
Since taking office on Friday, Mr Trump has signed a raft of presidential directives to begin action on a number of his campaign promises.
He confirmed he is pushing ahead with a border wall between the US and Mexico and that the cost to taxpayers would eventually be clawed back from Mexico.
In a live televised address to the nation, Mexico’s President Enrico Pena Nieto condemned the move and repeated that his country would not pay for the wall.
Mr Trump is expected to follow up with an order temporarily suspending the US refugee programme, ending arrivals from Syria, and refusing visas to people from a number of predominantly Muslim countries.
“This is not the Muslim ban,” he said but, according to the text of the draft order, it would refuse entry to anyone who could “bear hostile attitudes toward our country and its founding principles”.
Mr Trump also promised a “better plan and better access” to replace Obamacare and accepted that he had to reach out to the millions who marched in protest at his presidency.
“I have a responsibility to those who didn’t vote for Donald Trump,” he said.
Mr Trump also confirmed he had ordered an investigation into his claims that millions of fraudulent votes were cast in November’s election. The claim has been repeatedly debunked by officials.
The President said: “When you look at the people that are registered, dead, illegal and in two states, and some cases maybe three states, we have a lot to look into.
“I want the voting process to be legitimate.”
He again raised the issue of the size of the crowd at his inauguration and said he would not allow critics to “demean” those who travelled to Washington DC because they like him.
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