Mrs Clinton apologised for any offence her remarks may have caused
US presidential hopeful Hillary Clinton has apologised for remarks about Robert Kennedy's 1968 assassination as she defended her continuing nomination bid.
Senator Clinton said she had been attempting to point out that previous campaigns had also continued into June.
Democrat Robert Kennedy was running for his party's presidential nomination when he was shot dead in June 1968.
A spokesman for rival Democrat hopeful Barack Obama, whose safety has been an unspoken issue, criticised the remark.
Spokesman Bill Burton called the comments "unfortunate" and said they had "no place in this campaign".
Moment of trauma
Hillary Clinton cites shooting of Robert Kennedy as justification to stay in the US election race - and explains why she said it.
The comments came in a meeting Mrs Clinton was having with the editorial board of the Sioux Falls Argus-Leader newspaper.
Responding to those who had called on her to withdraw from the Democratic Party's presidential race, Mrs Clinton said: "My husband did not wrap up the nomination in 1992 until he won the California primary somewhere in the middle of June... We all remember Bobby Kennedy was assassinated in June in California. I don't understand it."
Mrs Clinton has made similar comments before, in March, without drawing criticism.
But analysts say the remarks could be damaging given the sensitivities over political assassinations, and fears for the safety of her rival - who began receiving Secret Service protection months before the primary campaign began.
The New York senator later expressed her regret at any offence her comments may have caused.
"I regret that if my referencing that moment of trauma for our entire nation - and in particular the Kennedy family - was in any way offensive. I certainly had no intention of that whatsoever."
In the race for the Democratic nomination, Mr Obama has so far won more of the delegates who will choose the party's candidate at a National Convention in August.
He is now just 56 delegates short of the number needed to clinch the nomination.
The next contest will be a primary in the US territory of Puerto Rico on 1 June, before the final two votes take place in Montana and South Dakota on 3 June.
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