Donald Trump clinches Republican nomination and moves one step closer to becoming US President

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

His triumph completes an unlikely rise that has upended the political landscape and sets the stage for a bitter Autumn campaign.

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

It takes 1,237 delegates to win the Republican nomination for president – Trump has reached 1,238. With 303 delegates at stake on June 7, Trump will easily pad his total, avoiding a contested convention this summer.

Many on the right have been slow to warm to Trump, wary of his conservative bona fides. Others worry about his crass personality and the lewd comments he's made about women.

But millions of grass-roots activists, many of them outsiders to the political process, have embraced Trump as a plain-speaking populist who is not afraid to offend.

Steve House, chairman of the Colorado Republican Party and an unbound delegate who confirmed his support of Trump, said he likes the billionaire's background as a businessman.

"Leadership is leadership," Mr House said. "If he can surround himself with the political talent, I think he will be fine."

Trump's pivotal moment comes amid a new sign of internal problems.

Hours before clinching the nomination, he announced the abrupt departure of political director Rick Wiley, who was in the midst of leading the campaign's push to hire staff in key battleground states.

In a statement, Trump's campaign said Wiley had been hired only on a short-term basis until the candidate's organisation "was running full steam".

Some delegates who confirmed their decisions to back Mr Trump were tepid at best, saying they are supporting him out of a sense of obligation because he won their state's primary.

Cameron Linton of Pittsburgh said he will back Trump on the first ballot since he won the presidential primary vote in Linton's congressional district.

"If there's a second ballot I won't vote for Donald Trump," Mr Linton said. "He's ridiculous. There's no other way to say it."