Republican presidential candidate Donald Trump has described as "rigged" the way the US state of Colorado picks its presidential nominee.

His rival Ted Cruz swept the board in that state, where delegates are selected by senior party activists.

Trump is still well ahead in the Republican race but if he does not get 1,237 delegates in the state-by-state contests, he could lose out anyway.

A winner could well emerge from a contested convention in July.

In that scenario, many of the delegates can back who they want to after the first ballot, opening the door for Texas Senator Cruz or even the third candidate in the race, John Kasich.

Senior Republicans have voiced concern about Trump's chances of winning in November's general election, and oppose his hardline views on immigration.

Next week New Yorkers get the chance to pick their nominees in the Republican and Democratic contests, in a pivotal moment in the process.

But on Monday, Trump, a New York businessman, told Fox News he was angry about the way Mr Cruz had won all the delegates in Colorado.

"They weren't given a vote. It's a crooked deal. The system is rigged, crooked. That's not the way democracy is supposed to work."

Delegates are party members with the power to vote for each candidate at the party conventions held in July, where the nominee is formally confirmed.

In the modern political era, most states have opted to hold state-wide primaries or caucuses to determine the number of delegates pledged to a particular candidate.

But Colorado decided to select its delegates in a different way, at its own state convention.

(BBC)