Republican White House hopeful John McCain said Friday that his Democratic rival Barack Obama is further to the left than the only Socialist US senator.

Asked by a reporter for the Kansas City Star newspaper if McCain, 71, believed Obama was an "extremist," McCain replied:

"That's his voting record. All I said was his voting record... is more to the left than the announced Socialist in the United States Senate, Bernie Sanders of Vermont," McCain said according to a video of the interview posted online.

"Do you think he's a socialist, Barack Obama?" the interviewer asked.

"I don't know," McCain answered.

Sanders was elected to the Senate in November 2006 after serving 16 years in the House of Representatives.

Sanders votes as an independent and calls himself a democratic socialist -- a title which has made him something of a phenomenon in US politics, where the word tends to carry a negative connotation more often linked to North Korea than to Scandinavian socialism.