Washington (AFP) - Former US senator Jim Webb of Virginia, a Vietnam war veteran who served as secretary of the navy, announced Thursday he is running for president, challenging frontrunner Hillary Clinton for the Democratic nomination.

"After many months of thought, deliberation and discussion, I have decided to seek the office of the presidency of the United States," Webb said in a Facebook post.

Webb's campaign is a long shot by any measure, but he is counting on appealing to voters from both sides of the aisle, his opposition to the invasion of Iraq, and his experience as a decorated US Marine to distinguish himself in the 2016 race.

"I understand the odds, particularly in today's political climate where fair debate is so often drowned out by huge sums of money," Webb said, adding that "more than one candidate" for president intends to raise $1 billion or more this election cycle.

"We need a president who understands leadership, who has a proven record of actual accomplishments, who can bring about bipartisan solutions, who can bring people from both sides to the table to get things done."

Webb, 69, is the fifth candidate seeking the Democratic nomination, after ex-secretary of state Clinton, Vermont's independent Senator Bernie Sanders, former Maryland governor Martin O'Malley, and former senator and governor Lincoln Chafee of Rhode Island.

Fifteen Republicans are officially seeking the their party's nomination, including former Florida governor Jeb Bush and conservative Wisconsin Governor Scott Walker.

Webb, a former Republican in swing-state Virginia, has expressed support for gun rights and opposition to coal plant regulations as well as most government programs that broadly favor non-whites.

Webb narrowly won his Senate seat in 2006. He declined to run for re-election in 2012.