Donald Trump has emerged the decisive winner of Nevada's Republican caucuses, leaving his main rivals battling for second place in an increasingly urgent effort to slam the brakes on the billionaire businessman's juggernaut.

Mr Trump now has three straight victories - in the West, the South and North east - a testament to his broad appeal among the "mad-as-hell" voters making their voices heard in the 2016 US presidential race.

Six in 10 caucus goers said they were angry with the way the government was working, and Mr Trump scooped about half of those, according to preliminary results of an entrance poll.

Nevada was a critical test for Marco Rubio and Ted Cruz, the two senators battling to emerge as the clear alternative to the Republican front-runner.

Mr Rubio was out to prove he could build on recent momentum, while Mr Cruz was looking for a spark to recover from a particularly rocky stretch in his campaign.

Already campaigning in Michigan as caucus results rolled in, Mr Rubio was projecting confidence that he could consolidate the non-Trump voters who have been splintering among an assortment of Republican candidates, saying: "We have incredible room to grow."

Mr Cruz, a fiery conservative popular among voters on the Republican right, finished a disappointing third in South Carolina after spending much of the past two weeks denying claims of dishonest campaign tactics and defending his integrity.

Another disappointing finish in Nevada would raise new questions about his viability heading into a crucial batch of Super Tuesday states on March 1, including his home state of Texas.

Nevada's caucusing played out in schools, community centres and places of worship across the state, a process that has been chaotic in the past.

Preliminary results of the entrance poll found that about three in 10 early caucus goers said the quality that mattered most to them in choosing a candidate was that he shares their values, slightly more than the quarter who said they want a candidate who can win in November.

About a quarter said they wanted a candidate who could bring change. About 2 in 10 want one who "tells it like it is".

Mr Trump is on a roll after winning primaries in New Hampshire and South Carolina. Mr Cruz won the lead-off Iowa caucuses.

Nevada marks the first Republican nominating contest in the West and the fourth of the campaign as the candidates try to collect enough delegates to win the party's nomination at the national convention in July.

Although Nevada has relatively few delegates, it is the first measure of voter sentiment in the vast western region, much as South Carolina was the first glimpse at the South's preferences last weekend.

Nevada is 28% Latino, 9% Asian-American and leads the nation with the highest rate of people living in the country illegally, according to the Pew Hispanic Centre.

Its immigrant communities - 19% of its population was born outside the United States - have helped turn a once reliably Republican state into one that backed Barack Obama twice. Many analysts attribute that to hardline Republican positions on immigration.

A Republican field that included a dozen candidates a month ago has been reduced to five, with former Florida governor Jeb Bush the latest to drop out after a disappointing finish in South Carolina.

Ohio governor John Kasich and retired neurosurgeon Ben Carson remain in the race and could play spoilers as the trio of leading candidates battle for delegates with an increasing sense of urgency.

Mr Trump's rivals concede they are running out of time to stop him. The election calendar suggests that if the New York billionaire's rivals do not slow him by mid-March, they may never. Mr Trump swept all of South Carolina's 50 delegates, giving him a total of 67 compared to Mr Cruz and Mr Rubio who have 11 and 10, respectively.

There are 30 delegates at stake in Nevada, awarded to candidates in proportion to their share of the state-wide vote so long as they earn at least 3.33%. 

A crowd of several hundred supporters gathered at Mr Trump's Nevada watch party at the Treasure Island hotel in Las Vegas and burst into cheers and screams at the news that he had won.

Some shouted "USA! USA!" as the caucus results rolled in. Many of the supporters in the crowd were from outside the state and decided to turn up at the celebration while in town.

Nevada caucus-goers who decided whom to support before the last week were key to Mr Trump's victory in the state, according to early results of the entrance poll conducted among people arriving at caucus sites.

Among those who decided whom to support in the last week, about four in 10 backed Mr Rubio, about a quarter supported Mr Trump and about two in 10 supported Mr Cruz.

But a majority of those deciding before the last week supported Mr Trump and they accounted for about seven in 10 caucus attendees.

The survey was conducted for The Associated Press and the television networks by Edison Research as Republican voters arrived at 25 randomly-selected caucus sites in Nevada.

The preliminary results include interviews with 1,545 Republican caucus-goers and have a margin of error of plus or minus four percentage points.