Hillary Clinton was forced on the defensive today after citing the 1968 assassination of Robert Kennedy as a justification to keep campaigning for president against Barack Obama.

In a filmed meeting with editors of a newspaper in South Dakota, which holds the final primary of the Democratic race on June 3, Clinton hinted that she might not concede the nomination to Obama until the August nominating convention.

"My husband did not wrap up the nomination in 1992 until he won the California primary somewhere in the middle of June, right?" Clinton said. "We all remember Bobby Kennedy was assassinated in June in California. I don't understand it."

Obama began travelling with guards from the US secret service last spring, far earlier than in previous elections. The ghastly spectre of threats on Obama's life was an early concern among some African-Americans who feared that voting for him could expose him to danger.

A furore erupted soon after videotape of her remarks became public, with Obama spokesman Bill Burton decrying the assassination reference as "unfortunate" and inappropriate for the campaign.

Clinton released a statement attributing her Kennedy reference to this week's brain cancer diagnosis of Democratic senator Edward Kennedy, the youngest of the US dynasty's three iconic brothers.

"The Kennedys have been much on my mind the last days because of Senator Kennedy and I regret that if my referencing that moment of trauma for our entire nation, and particularly for the Kennedy family, was in any way offensive," Clinton said. "I certainly had no intention of that whatsoever."

Robert Kennedy, the brother of Edward and President John F Kennedy, was killed by an assassin while campaigning as the frontrunner for the Democratic presidential nomination in California.

His death set off a period of intense turmoil in the US, with riots against the Vietnam war raging in many cities.

Earlier this year, the Clinton camp distanced itself from a New Hampshire supporter who referenced the Kennedy assassination while introducing the former first lady at a public event.

"Some people compare one of the other candidates to John F Kennedy," the New Hampshire supporter said while Clinton looked on. "But he was assassinated."

