Image copyright Reuters Image caption Florida-born Marco Rubio is the son of Cuban immigrants

Republican frontrunner Donald Trump has said he's "not sure" whether his rival, Florida Senator Marco Rubio, is eligible to run for US president.

Mr Rubio, who came second in the South Carolina primary, was born in Miami, Florida, to Cuban parents.

Texas Senator Ted Cruz, who was born in Canada, has faced similar questions from the New York billionaire.

Most legal experts believe the two senators meet the requirements to become president of the United States.

The US constitution allows only "natural born" citizens to become US president, which is widely interpreted as being born in the US or having a US citizen parent.

Mr Rubio, whose parents became US citizens four years after he was born, seemed to shrug off Mr Trump's comments, describing it as "a game he plays".

"I'm gonna spend zero time on his interpretation of the constitution with regards to eligibility and I'm gonna spend all my time talking about what this campaign should be about," the Florida senator told ABC News.

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Mr Trump was a vocal proponent of the "birther" movement questioning whether President Barack Obama was born in the US - a debate that continued even after Mr Obama published his birth certificate showing he was born in Hawaii.

The real estate mogul won the South Carolina primary on Saturday - his second electoral win in the Republican race for presidential nomination - by 10 percentage points, followed by Marco Rubio and Ted Cruz.

Both senators are trying to cast themselves as the one candidate for the Republican establishment to get behind and pose a serious challenge to Mr Trump.

Image copyright Getty Images Image caption Mr Obama released his birth certificate in 2011 to dispel rumours that he was not a "natural-born" US citizen

The frontrunner has led a colourful campaign marred in controversy, with threats to deport 11 million undocumented immigrants, build a wall on the southern border paid for by Mexico and stop Muslim immigrants from entering the country.

Questions about Marco Rubio's eligibility arose after Mr Trump retweeted a post at the weekend saying that both he and Mr Cruz were ineligible.

Asked by ABC's George Stephanopolous on Sunday about the post, Mr Trump said he was raising it to start a discussion on the matter but needed to look into it further.

"I don't know. I really - I've never looked at it, George. I honestly have never looked at it. As somebody said, he's not. And I retweeted it."

"I think the lawyers have to determine that," he added.

Candidates for US president must

be a "natural born citizen" - this has never been tested in court but is widely interpreted as being a US citizen at birth, so born in the US or having a US citizen parent

be 35 years of age or older

have lived in the US for the past 14 years

Mr Trump, whose own mother was born in Scotland, has previously threatened to sue fellow senator, Ted Cruz, "for not being a natural born citizen".

Mr Cruz, who was born in Calgary to an American mother and a Cuban father, has dismissed any such legal challenge arguing that the constitution's definition of "natural born citizens" included people born to an American parent.

Donald Trump has a history of questioning presidential candidate eligibility. In 2012, he repeatedly questioned President Obama's citizenship, prompting him to produce details of his birth records

Mr Obama was born in Hawaii to an American mother and a Kenyan father.

Mr Obama's allies have said the "birther" movement was a racist effort to discredit the county's first black president.