WASHINGTON (Reuters) - Former US Republican presidential candidate Marco Rubio on Sunday (May 29) ruled out becoming Donald Trump's vice presidential running mate, but said he'd be"honored" to play some kind of a role in helping him win the White House.

Rubio, who clashed bitterly with Trump in the brutal race for the Republican nomination, said his policy differences with the real estate mogul were too great for Rubio to join the ticket.

"I wouldn't be the right choice for him," Rubio said in an extensive interview with CNN's "State of the Union" programme."Donald deserves to have a vice president - he's earned the nomination - and he deserves to have a running mate that more fully embraces some of the things he stands for."

Rubio, a US senator from Florida, ended his White House bid in March after an embarrassing loss in his home state.

Trump, who has practically ensured he will emerge as the nominee from the Republican convention in July, derided Rubio as a lightweight and dubbed him "Little Marco."

Rubio called the real estate mogul a con artist and quipped about his small hands, a charge that Trump took to mean as questioning the size of his manhood.

Rubio and Trump differed sharply on policy issues, with Trump eschewing the interventionist approach favoured by Rubio and Rubio criticising Trump's call for temporarily banning the entry of Muslims into the United States.

Ahead of the July 18 to 21 Republican convention, Trump has sought to unify the party behind him and gain the backing of other prominent party figures.

Rubio said he expected to attend the convention and did not rule out a speaking role. He said he wanted to be helpful to Trump's presidential run because he wants to see likely Democratic nominee Hillary Clinton defeated in the Nov 8 US election.

"I don't want Hillary Clinton to be president. If there's something I can do to help that from happening and it's helpful to the cause I'd most certainly be honored to be considered for that," Rubio said.

In another olive branch to Trump, Rubio said he regretted making the "small hands" remark about his former foe. "I actually told Donald at one of the debates. I forget which one - I apologised to him for that," Rubio said.

"I said'I'm sorry that I said that.' It's not who I am. And I shouldn't have done it."