Hillary Clinton mentioned the June 1968 assassination of Robert Kennedy while explaining yesterday why she had resisted calls to end her White House bid.

The comments were criticised by Democratic front-runner Barack Obama's campaign.

Ms Clinton, who later expressed regret over the remark, made it to the editorial board of a South Dakota newspaper, the Sioux Falls Argus Leader, when explaining that other races for the Democratic presidential nomination had lasted into June.

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

'I don't understand it,' Ms Clinton said, referring to calls for her to pull out of the Democratic race in which the final nominating primaries are held on 3 June in South Dakota and Montana.

The Democratic nominee will face Republican John McCain in the November election.

Robert Kennedy, brother of US President John F Kennedy and Massachusetts Senator Edward Kennedy, was assassinated during his 1968 race for the Democratic presidential nomination.

Ms Clinton's comments drew a sharp response from Mr Obama's campaign who said her statement was unfortunate and had no place in the campaign.

Ms Clinton later said she regretted if referencing that moment was in any way offensive.

A spokesman said Ms Clinton had simply been pointing out that Democratic campaigns in the past had continued into June and therefore people should not be pressuring her to withdraw.

Click here to watch a clip from Hillary Clinton's interview