Washington: Donald Trump has reached the number of delegates needed to clinch the Republican nomination for US president, completing an unlikely rise that has upended the political landscape and set the stage for a bitter campaign.

The controversial businessman is also considering a public debate with independent Bernie Sanders in California that would sideline Democratic front-runner Hillary Clinton.

Despite Trump reaching the threshold of delegates, the Republicans' influential House Speaker, Paul Ryan, still declined to officially endorse the candidate on Thursday, only confirming at a press conference that the two had spoken again: "We had a very productive phone call, I'll leave it at that."

Mr Trump was put over the top in the delegate count by several news organisations on Thursday by a small number of the party's unbound delegates who confirmed they would support him at the national convention in July.