Donald Trump won the Nevada Republican caucuses by a huge margin, chalking up his third straight triumph in the party's presidential nominating process and building momentum ahead of a critical multi-state vote next week.

With about 90 percent of the votes counted, US media gave Trump about 46 percent, with senators Marco Rubio of Florida and Ted Cruz of Texas trailing by more than 20 points in a tight race for second.

"This is an amazing night," Trump told cheering supporters in a victory speech after Tuesday's vote.

"We weren't expected to win too much and now we're winning, winning, winning the country," Trump said.

"And soon the country is going to start winning, winning, winning."


The result underscored the enormous challenge Trump's rivals face as the candidates head into next week's all important "Super Tuesday" contests involving 11 states.

CNN and Fox News had Rubio in second place in Nevada with 23.7 percent of the vote and Cruz in third with 21.5 percent.

An ecstatic Trump said his win was broadly based, including strong support among Hispanics. He angered many early in the campaign by saying Mexico sends rapists across the border to the US.

"We won the evangelicals. We won with young. We won with old. We won with highly educated. We won with poorly educated," he said.

"I love the poorly educated. We're the smartest people. We're the most loyal people."


'Most important night' ahead

The remaining two candidates for the Republican presidential nomination, retired neurosurgeon Ben Carson and Ohio Governor John Kasich, lagged far behind in the single digits.

Cruz insisted he was the only candidate who could beat Trump and who has won a primary -- he prevailed in Iowa -- and said he was now setting his sight on next Tuesday's crucial contests.

"One week from today will be the most important night of this campaign," he said.

Trump had been all but certain to triumph in Nevada, with the big question being whether Rubio -- favoured by mainstream Republicans -- could clinch second place.

The contest was the fourth for the Republican presidential candidates, with Trump also winning in New Hampshire and South Carolina. He came second in Iowa.

Although the caucus in Nevada is not expected to have a significant impact on the overall race -- only 30 delegates or slightly more than one percent of the total are up for grabs -- it was the first contest for the Republicans in the US West.

It is also the first test of Republican voter sentiment after Jeb Bush pulled out of the race last week following a poor showing in South Carolina.

Trump expressed optimism that he might effectively clinch the nomination quickly.

"It's going to be an amazing two months," Trump said. "We might not even need the two months, to be honest."

All eyes were on whether Rubio and Cruz would be able to slow Trump's momentum and which of the two candidates would come in second.

Dan Lee, assistant professor of political science at the University of Nevada, Las Vegas, said the latest caucus came as mainstream Republicans are grudgingly accepting the fact that Trump may well end up the party's nominee given his seemingly unstoppable winning streak.

"A lot of Republicans -- especially the Republican establishment, professionals, governors -- don't really want Trump to win the nomination," Lee said.

"They want to get Cruz out and have Rubio go against Trump."


'Soft, weak, little baby'

The real estate magnate dished out his trademark rhetoric against his rivals ahead of the vote, comparing Cruz to a "soft, weak, little baby" at a rally.

"But for lying, he's the best I've ever seen," he added.

Cruz fired back, accusing Trump of consistently vacillating on issues and saying his insults showed how rattled he was.

"@realDonaldTrump, showing class & grace, calls me a 'soft weak little baby,'" Cruz tweeted. "Hope he doesn't try to eat me!"

The Republican field, which once stood at 17, has shrunk to five, with Bush the latest to pull out on Saturday.

After Nevada, the real test on where the presidential candidates stand will come on March 1, when 11 states go to the polls in what is known as "Super Tuesday."

Unlike primaries, caucuses allow participants to openly engage with one another and hear arguments from candidates' supporters or surrogates, in meetings at schools, community centers and churches.

Republicans then vote by secret ballot, in 130 caucus sites across Nevada.

The results will be used to determine the number of Republican delegates who represent the state at the party's nominating convention in July.