Race enters key third round

The contest for Republican and Democratic contenders to become presidential candidates in the United States has entered its third round.

Polls have already opened in the Republican primary vote in South Carolina, where frontrunner Donald Trump will try to fend off Ted Cruz.

Hillary Clinton and Bernie Sanders could face a close race in the Democratic Party caucus in Nevada.

The votes could be key ahead of the "Super Tuesday" round on 1 March. On that day, about a dozen states will choose their candidates for the 8 November presidential election, with about a quarter of all nominating delegates up for grabs.

The leading Republican pair split the first two rounds - Trump winning in New Hampshire, a week after Cruz had triumphed in Iowa.

Cruz has enlisted an army of volunteers to try to haul back Trump's lead in the state - put at about 28% to 23% in an NBC-Wall Street Journal poll of Republican voters. That poll suggested Senator Marco Rubio was third on 15%, followed by Jeb Bush on 13%.

In Nevada, Democrats will gather at about 200 caucus meetings across the state from 11:00 local time (19:00 GMT) to show their support for Clinton or Sanders.

The pair also split the first two rounds - Clinton winning narrowly in Iowa, before being soundly beaten in New Hampshire.

Sanders' socialist message has generally energised the youth vote and a win here, or even a narrow loss, could be a huge boost ahead of Super Tuesday.