Samantha Power, the U.S. ambassador to the United Nations, said in an interview broadcast Thursday that she regrets calling Hillary Rodham Clinton a "monster" during the 2008 presidential campaign.

"I regretted it pretty much every day since," Power told NBC's "Today."

A foreign policy adviser on then-Sen. Barack Obama's campaign, Power told a newspaper Clinton "is a monster" and "is stooping to anything." She soon resigned from the campaign over the remarks.

"It just completely broke my heart that there is a fair amount of negativity heaped upon her that I find massively unfair," she told "Today." "And the idea that I could have contributed in some way to that narrative is just terrible."

Power said apologizing to Clinton in person was "very emotional" for her.