Trump has the numbers to secure Republican nomination - AP

Donald Trump (Reuters file)
Donald Trump (Reuters file)

Donald Trump has secured the delegate numbers he needs to clinch the Republican Party nomination for United States President, the Associated Press (AP) says.

He was put over the top by a small number of the party's uncommitted delegates who say they will support him at the party's national convention in July.

The AP says this brings Mr Trump's delegate count to 1238 – one more than needed.

Meanwhile, Mr Trump and second-placed Democrat Bernie Sanders could face off in an unconventional debate in California, in the latest twist to an unusual campaign.

The two candidates expressed an interest in squaring off in a one-on-one encounter that would leave Democratic front-runner Hillary Clinton on the sidelines.

Mr Sanders, a US senator from Vermont, is running behind Ms Clinton in the race for the Democratic presidential nomination.

Republican and Democratic presidential candidates traditionally do not debate each other until the parties have selected their respective nominees.

In an appearance on ABC's Jimmy Kimmel Live late on Wednesday, billionaire Mr Trump said he was willing to participate in a debate with Mr Sanders, a democratic socialist.

"If I debated him, we would have such high ratings," the former reality TV star said. "I think I should ... take that money and give it to some worthy charity."

Mr Sanders appeared to agree in a post on Twitter on Thursday.

"Game on," he tweeted. "I look forward to debating Donald Trump in California before the June 7 primary."

The hashtag #BernieTrumpDebate began trending in the United States with news of the possible debate.

Mr Trump's spokeswoman Hope Hicks said in an email on Thursday that there were no formal plans yet for such an event.

But Mr Sanders' spokesman Michael Briggs said the campaign plans to reach out to the Trump camp to discuss the idea.

"We look forward to following up and making sure it happens," Briggs said.

Kimmel said he asked Mr Trump about the debate at the suggestion of Mr Sanders, who is scheduled to appear on the show Thursday night.

Mr Sanders, who has made economic equality a keystone of his campaign, has criticised Ms Clinton for backing out of an agreement to debate him before the California primary.