US House Speaker Paul Ryan has officially ruled out making a late attempt to become the Republican presidential nominee.

"I do not want, nor will I accept the Republican nomination," he said.

Ryan's name was floated as a late contender if there is a contested convention in July, as doubts persist over the strength of the candidates.

If Donald Trump, John Kasich nor Ted Cruz is able to win 1,237 delegates, the convention will be contested.

The state-by-state primary contests, which come to New York next week, determine the number of delegates pledged to a particular candidate.

Trump is still well ahead in the number of delegates accumulated but may fall short of the magic number required.

At a contested convention, the delegates are free after the first ballot to back whom they want, opening the door for Texas Senator Cruz or even the third candidate in the race, Kasich.

Some in the party had hoped Ryan would emerge as a candidate at that stage, believing he would be a more effective and less divisive figure than Trump or Cruz.

Speaking at the Republican National Committee headquarters in Washington, Ryan - who ran as Mitt Romney's running mate in the 2012 presidential election - ruled himself out unequivocally.

But some commentators were quick to point out that he said he did not want to run for Speaker of the House last year before eventually accepting the job.