UPDATED 8.22am Donald Trump has reached the number of delegates needed to clinch the Republican nomination for US president.

LISTEN ABOVE: Howard Fineman talks to Mike Hosking

Trump was put over the top in the Associated Press delegate count by a small number of the party's unbound delegates who said they would support him at the convention.

Among them is Oklahoma GOP chairwoman Pam Pollard.

"I think he has touched a part of our electorate that doesn't like where our country is," Pollard said. "I have no problem supporting Mr Trump."

It takes 1237 delegates to win the Republican nomination for president. Trump has reached 1238.

With 303 delegates at stake in five state primaries on June 7, Trump will easily pad his total, avoiding a contested convention in Cleveland in July.

Trump's riding high.

"If you look at what's happened, tremendous support from all over the country. Senators, congressman, we have Governors all over the place and vast majorities. I think the approval now is up to over 90 percent. That's tremendous from where I started, a little while ago it was 62 per cent."

Huffington Post journalist Howard Fineman told Mike Hosking history shows it's unlikely the Democrats will hold power for a third straight term so it's in Trump's favour.

"So Hillary has history against her and Americans love novelty and Donald Trump is, if nothing else, novel."

The news comes after President Barack Obama that foreign leaders were "rattled" by Trump and had good reason to feel that way, accusing the property magnate of being ignorant about world affairs.

Obama's assessment of the presidential campaign came on the sidelines of a Group of Seven advanced economies summit in Japan earlier this week, the latest world gathering to be coloured by global concerns about Trump.

Obama said foreign leaders at the conference were "surprised by the Republican nominee" and unsure how seriously to take his pronouncements.

"They are rattled by it - and for good reason," Obama said. "Because a lot of the proposals he has made display either ignorance of world affairs, or a cavalier attitude, or an interest in getting tweets and headlines."

He contrasted that to proposals that thoughtfully address what's required to keep the US safe and prosperous and "to keep the world on an even keel."

Trump came back saying: "He is a man who shouldn't be really airing his difficulties, and he shouldn't be airing what he's airing where he is right now and I think you are going to see it stop very soon. He has not done a good job.

"He's a president who's allowed many of these countries to totally take advantage of him, and us unfortunately, and he's got to say something. It's unusual that every time he has a press conference he's talking about me.

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

In a news conference, Obama brushed off calls for Hillary Clinton and Bernie Sanders to move hurriedly to resolve the primary so that Democrats can unite behind one candidate, arguing that unlike the Republicans, this year's Democratic candidates weren't that ideologically divided.

He likened the hard-fought campaign between Clinton and Sanders to the one he waged with Clinton in 2008.

"During primaries, people get a little grumpy with each other," Obama said. "Somebody's supporter pops off and there's a certain build-up of aggravation. Every little speed bump, conflict trash-talking that takes place is elevated."

He urged both Democratic candidates to "try to stick to the issues," adding that the grumpiness often stemmed from voters' frustration when the campaign instead becomes dominated by talk about "personalities and character.

Trump addressed the terrorism problem saying it's a worldwide issue and not enough is being done to stop it.

"We have a radical Islamic terrorism problem. It's a worldwide problem, not just this country. We have to find a solution, and we have to be vigilant, and we have to be tough and smart."

Hillary Clinton, who looks likely to be Mr Trump's Democratic opponent, is close to securing the delegate numbers she needs to win her party's nomination.

Mr Fineman said it would take a huge scandal for Clinton to lose the support of her delegates.

"Such as an FBI report on the emails or something like that, that would come close to putting her in legal jeopardy to turn that around. I don't see that happening."

Americans will head to the presidential polls on November 8th.