Donald Trump and Hillary Clinton remained the front-runners in the quest for the White House, but Senators Ted Cruz and Bernie Sanders scored key victories in the latest contests.

Republican Trump and Democrat Clinton both won in the delegate-rich state of Louisiana on Saturday, keeping them on top at a critical point in the US presidential race.

Results from votes in five states were split, but one element was clear: Republican Cruz boosted his claim as the most viable alternative to Trump after taking Kansas and Maine and put poorly performing Senator Marco Rubio under immense pressure to turn his campaign around or bow out.

And two Sanders victories showed that he can stay in the race despite the long odds of defeating Clinton.

Cruz and Sanders can also claim momentum as they head toward critical races in Michigan next Tuesday.

"Thank you to Louisiana, and thank you to Kentucky," Trump said in Florida, minutes after he won in Kentucky, where he led Cruz by four percentage points.

The races were the first test of whether the Republican establishment's desperate effort to halt Trump, led this week by 2012 nominee Mitt Romney, is having any effect.

Trump declared those efforts a failure, and called on Rubio, once seen as the best hope to defeat him, to quit.

Trump is well ahead in the all-important delegate count for Republicans, having now won 12 of the 19 states that have voted up to now.

But the strong showing by Cruz - who won more delegates than Trump on Saturday - indicates that the brash real estate mogul is not the inevitable nominee.

The Texas senator performed beyond expectations in Kansas, where he earned 48.2 percent support, doubling up on Trump who received 23.3 percent.

Rubio was third at 16.7, followed by Ohio Governor John Kasich with 10.7 percent.

AFP - AP