SAN JUAN (AFP) - Florida Senator Marco Rubio won the Republican primary election in Puerto Rico on Sunday (March 6), in a White House race otherwise led by billionaire Donald Trump.

Mr Rubio drew 71 per cent of the vote, ahead of Mr Trump (13 per cent), ultraconservative Texas Senator Ted Cruz (eight per cent) and Ohio Governor John Kasich (one per cent), the electoral board said in announcing the final results.

Residents of this US commonwealth in the Caribbean have American citizenship but cannot vote in the US presidential election if they are Puerto Rico residents. Still, they take part in the primary process.

Mr Rubio's victory hands him the 23 party convention delegates in play.

His campaign was in need of some good news. The son of Cuban immigrants who grew up in Las Vegas and Florida has been struggling in the presidential race.

If confirmed, this would mark just the second outright win for Mr Rubio, who also took Minnesota on Tuesday.

Mr Trump has kept a firm grip on his lead in the Republican race, and called for Mr Rubio to end his presidential bid.

Senator Ted Cruz has emerged as Mr Trump's strongest challenger in weekend primaries with mixed outcomes.

Mrs Hillary Clinton, meanwhile, extended her front runner status in the Democratic contests, though Senator Bernie Sanders showed he is still in the race with a few victories.

Mr Sanders was projected to win Maine's nominating contest.

Mrs Clinton and Mr Sanders also faced off in a televised debate in Flint, Michigan just two days before a crucial primary in that delegate-rich northern industrial state.