Mr Trump thanked the North Dakota delegates who pushed his supporter numbers past the amount required. Photo: Reuters

In North Dakota on Thursday, he thanked 15 unbound delegates from the state who he said "got us right over the top".

He defeated 16 other Republican contenders and according to the Associated Press has 1,238 delegates, one more than needed.

Republicans will finalise their nomination at a convention in July.

While Mr Trump has the required amount of delegates, his nomination by a divided Republican Party is not yet secured.

If confirmed, Mr Trump will face former US Secretary of State Hillary Clinton or Vermont Senator Bernie Sanders, who are vying for the Democrat nomination.

On Wednesday, the New York billionaire suggested going against Mr Sanders in a TV debate in California before the state's primary on 7 June.

Mr Sanders agreed to the debate in a tweet, saying "Game on".

On Thursday, Mr Trump said: "The problem with debating Bernie? He's going to lose."

He also threw a barb in Mrs Clinton's direction, saying: "Here I am watching Hillary fight and she can't close the deal. That should be such an easy deal to close."

Earlier, the current US president Barack Obama said that world leaders "had good reason to be rattled" by Mr Trump, whose proposals he said were "either ignorance of world affairs or a cavalier attitude".

In response to that, Mr Trump told reporters in North Dakota that rattling leaders of other countries was a "good thing".

"[President Obama] knows nothing about business," he said.

"Many of the countries in our beautiful world have been absolutely abusing us and taking advantage of us.

"We're going to have great relationships with these countries but if they're rattled in a friendly way that's a good thing, not a bad thing."