A supporter wearing a "Chinese Americans Love Trump" shirt takes the stage with other female Trump supporters at a campaign rally for the presumptive Republican presidential candidate on Wednesday in Anaheim, California. Photo: AFP

US President Barack Obama said Thursday that global leaders are "rattled" by some of presumptive Republican presidential nominee Donald Trump's policies, blasting the ideas as demonstrating "ignorance" of how the world works.

Trump on Thursday secured the support of enough delegates to clinch the Republican presidential nomination, after unbound delegates pledged to back the billionaire, according to the Associated Press count.

The Republican Party will not make the results official until its national convention in July, when delegates actually cast their votes for the nominee.

Trump, the billionaire US real-estate mogul and reality TV star, has dominated headlines since launching his presidential campaign last year with a mix of incendiary comments and policy stances seen as insulting Mexicans, Muslims and women, among others.

He has proposed building a giant wall along the US border with Mexico to keep out illegal immigrants and vows that he will get Mexico City to pay for it.

Trump has also proposed a temporary ban on Muslim immigration to the US, citing fears of jihadist attacks such as those that have occurred in Europe and the US city of San Bernardino.

"They are not sure how seriously to take some of his pronouncements," Obama told reporters on the sidelines of the Group of Seven summit at Japan's Ise Grand Shrine, referring to global leaders.

"But they're rattled by them, and for good reason," he added.

"Because a lot of the proposals that he has made display either ignorance of world affairs or a cavalier attitude, or an interest in getting tweets and headlines."

He has disdained the usual caution of past Republican and Democratic candidates for the country's highest office, who - while still appealing to their base constituencies - have often tried to take positions closer to the political center in order to appeal to broad cross sections of the electorate.

Obama is coming to the end of this second term and hopes to pass the mantle to fellow Democrat Hillary Clinton, his 2008 rival for the White House who also served as his secretary of state.

But recent opinion polls show that Clinton and Trump would be locked in an increasingly tight race, were they to meet in November's election after winning formal nominations at their party conventions this summer.

Clinton is vying to become the first female president in US history, and while she still has a Democratic primary race lead over challenger Bernie Sanders, the Vermont Senator has remained competitive and has drawn significant support from younger voters.

Clinton's camp has grown increasingly frustrated with the tenaciousness of Sanders, who has vowed to take his fight to the convention even as Clinton continues to hold a higher number of convention delegates, who decide the nomination.