US presidential hopefuls Donald Trump and Hillary Clinton have maintained their status as front-runners in the latest round of voting for their parties' nominations.

Despite defeats to Ted Cruz in Kansas and Maine, Republican Trump won in Louisiana and Kentucky.

Other Republican contenders Marco Rubio and John Kasich made little progress.

Clinton lost to Bernie Sanders in the Democratic races in Kansas and Nebraska, but won in Louisiana.

Bernie Sanders took Kansas and Nebraska - but Hillary Clinton won big in Louisiana. Photo: Reuters

Texas Senator Cruz now appears to be the only candidate who can stop Trump, analysts say, after a week in which the Republican establishment did everything it could to attack the New York tycoon.

Observers also point out that Cruz and Vermont Senator Sanders both won in caucuses but lost in the Louisiana primary elections, involving far more voters.

The Democratic race in Louisiana - comfortably won by Clinton - also appears to have exposed Sanders' lack of support among African-American voters.

Both Trump and Cruz urged the other Republican contenders to drop out of the race.

Speaking after his wins in Kentucky and Louisiana, Trump told a news conference: "I would love to take on Ted Cruz one on one."

Cruz suggested it was time for Senator Rubio and Ohio Governor Kasich to step aside.

"As long as the field remains divided, it gives Donald an advantage," he said.

Clinton said she was thrilled to add to her delegate count.

She added: "No matter who wins this Democratic nomination, I have not the slightest doubt that on our worst day we will be infinitely better than the Republicans on their best day."