WASHINGTON (AFP) - Former United States president Bill Clinton attributed outspoken Republican presidential hopeful Donald Trump's current success in surveys to his skills as a "master brander", but said his appeal could be short-lived.

Speaking late on Tuesday on the CBS 'Late Show,' host Stephen Colbert asked Mr Clinton to explain why he thought that Mr Trump - currently leading a pack of 15 Republican presidential hopefuls - was doing so well.

"He's a master brander, and he's the most interesting character out there... and because he says some things that overrides the ideological differences," Mr Clinton said.

"There is a macho appeal of saying I'm just sick of nothing happening. I make things happen. Vote for me."

Mr Clinton however said that Mr Trump's success "may have a short shelf life".

The former president (1993 to 2001) denied a rumour that he urged Mr Trump to run for president.

"His daughter told my daughter that he had tried to call me and I didn't get the message. So I simply called him back," said Mr Clinton. "I had a very pleasant conversation with him and it wasn't about running for office."

Mr Clinton's daughter Chelsea is a close friend of Mr Trump's daughter Ivanaka, according to US news media reports.

And who does he think is the most qualified to be elected president next year?

"The lady I saw singing on 'Saturday Night Live,'" he answered.

That would be his wife, former secretary of state Hillary Clinton, who appeared on the comedy TV show on Saturday.