WASHINGTON (AFP) - One of America's most liberal senators, Vermont's Bernie Sanders, said on Wednesday that he will seek the Democratic presidential nomination, presenting a challenge to Hillary Clinton from the party's left flank.

The 73-year-old Sanders, an independent who caucuses with Democrats but has described himself as a "socialist," told USA Today newspaper and the Associated Press that he will seek the party's presidential nomination for 2016.

Sanders' entry into the race is not likely to pose a serious threat to Clinton's frontrunner status, but could force her to clarify her positions on hot-button issues such as free trade deals currently under negotiation, which the party's left wing and unions oppose.

"Are you on the side of working people who would suffer as a result of this disastrous trade agreement, and seeing their jobs go to China or Mexico, or are you on the side of corporate America?" Sanders asked of Clinton during a recent interview on CNN.

The ties of Clinton and her husband - former president Bill Clinton - to Wall Street have given some, especially Sanders, pause. "I do have doubts about whether Hillary Clinton or whether any Republican candidate out there is prepared to take on the big money interests who control so much of our economy," Sanders told Fox News earlier this month.

Clinton, he said, will have to convince the American people that, despite her past record, "she is going to break up the major banks on Wall Street." Sanders will hold a major campaign kickoff in several weeks, Vermont Public Radio has reported.

He was elected to the US House of Representatives in 1990 as an independent from his north-eastern state. In 2006, he won a seat in the Senate, and was re-elected in 2012.

Clinton, the only Democrat to have officially announced her candidacy so far, signaled her intent to run earlier this month. RealClearPolitics has reported that 62 per cent of Democrats prefer Clinton, against a paltry six per t for Sanders, whose national profile is rather muted.