Donald Trump has said he knows who was behind the al-Qaeda terror attacks on the US in September 2001 and added that “Iraq did not knock down the World Trade Centre”.

“It was not Iraq,” the president told ABC News. “It were other people. And I think I know who the other people were. And you might also.”

Almost 3,000 people died when 19 terrorists - most of them Saudis - hijacked four passenger planes on 11 September, 2001. Two of the planes were crashed into the Twin Towers in New York City, one into the Pentagon in Virginia and one was brought down in Pennsylvania after passengers fought back.

The al-Qaeda terror network, led by Osama bin Laden, claimed responsibility for the atrocity.

In an interview with ABC’s George Stephanopoulos, Mr Trump criticised America’s military intervention in the Middle East, which he said was ”the worst decision made in the history of our country”.

He described the region as “like quicksand” and added: ”It was a terrible decision to go into the Middle East. Terrible.

“We’re now up to almost $8tn (£6.4tn). And when we want to build a roadway, a highway, a school, or something, everyone’s always fighting over money. It’s ridiculous. So that was a bad decision.”

He added: “And, by the way, Iraq did not knock down the World Trade Centre. It was not Iraq. It were other people. And I think I know who the other people were. And you might also. But it was a terrible decision.”

The US, backed by allies including Britain, invaded Afghanistan, where the terror group was being sheltered. But 9/11 was also used as part of the justification for the 2003 invasion of Iraq, which resulted in the deaths of hundreds of thousands of Iraqis, more than 4,000 Americans and 179 British troops, and contributed to the rise of the Isis terror group.

Mr Trump made the remarks after he was asked why he praised Kim Jong-un, the leader of North Korea.

The president initially claimed he says “good things about lots of people” before remarking “I’ve done a great job” and saying “everyone thought we were going to be at war” the day after he was elected.

He then said he had been “rebuilding the military” which he said was “totally depleted between President Bush with the Middle East” and then segued into his attack on the “terrible decision to go into the Middle East”.

Donald Trump inaccurately claims New York Stock Exchange was open the day after 9/11

Mr Trump has made several controversial remarks about the attacks on the Twin Towers.

During a Republican primary debate, the then-candidate blamed Bill Clinton and George Bush for not killing Osama bin Laden.

He said: “The World Trade Centre came down because Bill Clinton didn’t kill Osama bin Laden when he had the chance to kill him. And George Bush... by the way, George Bush had the chance, also, and he didn’t listen to the advice of his CIA.”

After the attacks, Mr Trump claimed ”thousands and thousands of people were cheering” the day the towers fell and later added that the cheers came from areas of New Jersey with “large Arab populations”.

His allegations were denied by police and Steven Fulop, the Jersey City mayor.