COLUMBIA, United States (AFP) - White House hopefuls Hillary Clinton and Bernie Sanders were in a tight race on Saturday in Nevada's Democratic caucuses, early results showed, while Donald Trump looked for a big win in the South Carolina Republican primary.

The two main US political parties parted ways for the third stage of the surprise-filled presidential nominations race, with the Democrats heading west and the Republicans in the south.

In Nevada, Democratic voters streamed into caucus sites to show their support for either Clinton or rival Bernie Sanders. With nearly 39 per cent of precincts reporting, Clinton was at 51.4 per cent to 48.5 per cent for Sanders.

Voters meanwhile streamed to the polls in South Carolina, in what could be an important test of strength for the 69-year-old frontrunner Trump. Voting places were to close at 7pm (8am on Sunday, Singapore time).

In the desert state of Nevada, both Clinton and Sanders have been working hard to reach out to the African-Americans, Hispanics and Asian-Americans who make up roughly half of the state's population.

Clinton, who won by a hair in Iowa but was crushed by Sanders in New Hampshire, is counting on a major Hispanic voter turnout, especially among Las Vegas hotel and casino employees.

Nevada has some three million residents, and the population is overwhelmingly concentrated in two large urban centres, Las Vegas and Reno.

The former top US diplomat also needs a win in Nevada, a state once seen as a relatively easy victory for her - one where her advantage has dissipated as the Sanders campaign gathered steam after trouncing Clinton in New Hampshire on Feb 9.

Since Wednesday, the 68-year-old Clinton has visited staff at casinos in Las Vegas, where workers "caucused" right on the famous Strip.

"I need your help this morning - in the show room, 11am," she told employees at Harrah's on Saturday, less than an hour before caucus time. Sanders visited the same casino cafeteria about 20 minutes earlier.

Clinton says she is the natural ally of Latinos on immigration, and if elected she promises a quick path to citizenship for those in the country illegally.

The former first lady and senator from New York has relentlessly attacked Sanders for voting against immigration reform in 2007.

Sanders counters that the measure gave little protection for foreign "guest workers," and that he voted for a 2013 immigration reform Bill that died due to Republican opposition.

Clinton claims that Sanders is offering impractical, pie-in-the-sky ideas, but Sanders' camp is convinced that young minority voters will back him.

In South Carolina, pre-vote surveys showed the billionaire businessman Trump with a lead over his five Republican rivals, although his advantage had shrunk in one poll in the lead-up to Saturday's primary in the Palmetto State.

Lynn Derrick, a regional vice-president for Oracle Corporation and a first-time primary voter, said he had cast his ballot for Trump.

"I want to put somebody in the White House who has a business background," Derrick told AFP at a high school in the state capital Columbia.

"We just think we want to take a risk with Trump. We think he's had success with everything he's touched."

Trump is banking on a big symbolic win ahead of "Super Tuesday" on March 1, when about a dozen states will go to the polls to choose candidates for the Nov 8 presidential election, with a quarter of the nominating delegates up for grabs.

"It's crunch time, folks," Trump told voters at a North Charleston rally, his final pitch Friday before the primary.

He finished second to Texas Senator Ted Cruz in Iowa on Feb 1, but secured a commanding win in New Hampshire one week later. The two have duked it out in the past week, with the campaign growing increasingly nasty.

On the eve of the primary, Trump led with about 28 per cent of likely Republicans voters backing him, according to an NBC-Wall Street Journal poll.

The ultra-conservative Cruz followed with 23 per cent. Trailing were Senator Marco Rubio at 15 per cent and former Florida governor Jeb Bush at 13 per cent.

Rubio and Bush are under intense pressure to fare well Saturday, as are Ohio Governor John Kasich and retired neurosurgeon Ben Carson, whose campaigns have struggled.

Edwin Pearlstine, a retired beer distributor and brewery owner who said he voted for Kasich even while conceding he had little chance of winning, said if Trump wins the Republican nomination, he'll leave the country.

"I got a pretty place right on the beach in the Bahamas. I'll just go stay there," he said.