Donald Trump and Hillary Clinton have each scored decisive wins after counting of primary contests in five critical states in the US nomination races.

Mr Trump swept all delegates in the Republican primary in Florida, a result that prompted one of his major rivals, Senator Marco Rubio, to suspend his campaign.

The Republican frontrunner will increase his delegate lead over main rival Ted Cruz, but doubts remain on whether he will be able to gather a majority of delegates in his own right before the Republican convention in Cleveland in July.

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The controversial real estate magnate lost the important Ohio primary to that state’s governor, John Kasich. 

The result has revived Mr Kasich’s campaign, adding another twist to the Republican Party’s tortuous primary race.

Gone … Rubio suspends his campaign after Florida loss. Photo: AAP

National opinion polls taken since February show Mr Kasich as the most likely Republican candidate to beat Hillary Clinton in a presidential election.

“This is about pulling us together, not pulling us apart,” he told supporters in a victory speech.

“I have to thank the great people of Ohio, I love ya.”

Despite the loss of Ohio, Mr Trump also won North Carolina and Illinois to more than double his lead over Mr Cruz in the race for the party’s nomination.

Mr Trump is also ahead by a small margin in Missouri, although Mr Cruz refused to concede defeat before counting was adjourned.

Perhaps the biggest winner in the latest round of primaries was Democratic frontrunner Hillary Clinton, who has almost shut the door on Vermont senator Bernie Sanders’ hopes of securing the party’s presidential nomination

Mrs Clinton won four states – Florida, North Carolina, Ohio and Illinois, with senator Sanders narrowly winning Missouri.

The nomination battles move next week to Arizona and Utah where around 100 delegates are up for grabs for the Republican candidates and 150 are on the line for Democrats.