Americans from New Jersey to California cast ballots Tuesday in the final major primaries of the 2016 White House race, after delegate counts showed Hillary Clinton clinching the Democratic nomination.

The results are expected to see Clinton declare victory as the first woman in history to secure the presidential nomination of a major US party, putting her on course for a showdown with New York billionaire Donald Trump, the presumptive Republican nominee, in November's election.

Her challenger Bernie Sanders has so far ignored media reports that she has clinched the magic number of delegates. The self-declared democratic socialist is chasing a win in the country's largest state California, and insisting the nominee will not be chosen until the party convention in July.

Voting was underway in six states - California, New Jersey, New Mexico, Montana, North Dakota and South Dakota. New Jersey and the Dakotas will be the first polls to close at 8:00 pm Eastern time (0000 GMT Wednesday). It remained unclear what effect the reports that Clinton had passed the milestone of 2,383 delegates Monday would have on those casting ballots.

"I'm a Bernie Sanders guy all the way." said 21-year-old student Saul Gomez, voting in Los Angeles. "Honestly he seems a lot more genuine to me," he said. "He's consistent, and I dig that." Clinton - the 68-year-old former first lady, New York senator and for four years President Barack Obama's top diplomat - is scheduled to deliver a speech in Brooklyn late Tuesday in perhaps the pinnacle of her political career to date. It marks a dramatic resurgence for the woman often described as the most experienced US presidential candidate in generations, but who remains mired in scandal about her emails and perceptions of dishonesty - leaving her with unpopularity ratings surpassed only by the controversial Trump.

Her stiff campaigning style saw her lose to Obama in their 2008 nomination battle and has drawn stark contrast to the crowds of thousands of young people that 74-year-old Sanders has energized nationwide. Clinton has refrained from getting too carried away by the US network tallies showing her capturing the nomination, urging her supporters to still come out and vote.