An adviser to Barack Obama who called Hillary Clinton a "monster" during an interview has resigned, apologizing for the remark amid pressure from the Clinton campaign.

Samantha Power, an unpaid foreign policy adviser to Obama and Pulitzer Prize-winning writer, made the offending comment during an interview with the Scottish newspaper The Scotsman.

"With deep regret, I am resigning from my role as an adviser to the Obama campaign effective today," Power said in a statement Friday. "Last Monday, I made inexcusable remarks that are at marked variance from my oft-stated admiration for Senator Clinton and from the spirit, tenor, and purpose of the Obama campaign. And I extend my deepest apologies to Senator Clinton, Senator Obama, and the remarkable team I have worked with over these long 14 months."

Power had first released an apology, saying her remarks "do not reflect my feelings about Senator Clinton."

Clinton praised Obama for the campaign response, but then tried to get mileage out of another Power comment -- her campaign circulated a Politico.com story Friday that referenced an interview Power gave to BBC, where she said Obama's plan to get combat forces out of Iraq in 16 months is a "best-case scenario."

"He keeps telling people one thing, while his campaign tells people abroad something else," Clinton said. "I'm not sure what the American people should believe."

Before Tuesday's election -- where she won Texas, Ohio and Rhode Island -- Clinton criticized Obama over a report on a Canadian news network saying his advisers were floating the word that Obama's threats to renegotiate NAFTA were just political rhetoric.

Obama later affirmed that he would bring the war "to an end in 2009."

"Don't be confused," he said in Wyoming. "When Senator Clinton is not even willing to acknowledge that she voted for war ... I don't want to play politics on this issue because she doesn't have standing to question my position on this issue."

Power's controversial comments to The Scotsman came during what the newspaper described as an "unguarded moment," as Power was describing how the Clinton campaign was stopping at nothing to catch up to Obama in the Democratic race.

"She is a monster, too -- that is off the record -- she is stooping to anything," Power told the newspaper, which published the remarks in Friday's edition and released them earlier on the paper's Web site.

In saying it was "off the record," Power tried to withdraw her remark, but the newspaper explained at the bottom of its story that the interview with Power, who was promoting her book, was established as "on the record" in advance. The newspaper wrote that it was "too late" for her to retract her comment.

Power also told the newspaper Clinton was appealing to low-income voters by trying to suggest Obama would endanger their jobs.

"The amount of deceit she has put forward is really unattractive," she said.

Clinton supporters on a conference call early Friday called for Power to leave the campaign.

"Sen. Obama has talked about a new kind of politics. This is the worst kind of politics," New York Rep. Gregory Meeks said. "This is personal character assassination."

Clinton spokesman Howard Wolfson noted that those involved in the Clinton campaign had been removed when they spoke of Obama's teenage drug use or helped spread the false rumor that the Illinois senator is a Muslim.

He defended his own comparison of Obama to independent prosecutor Kenneth Starr, saying he'd been responding to "attacks" from the Obama campaign regarding Clinton's tax returns and real estate transactions. That, he said, was a clear reference to Whitewater and so it was appropriate to bring up Starr in that context.

Later, after Power resigned, the Clinton campaign sent a fundrasing e-mail to supporters pointing out the "monster" quote without mentioning she left the campaign. "A small contribution now -- even as little as $5 -- will show the Obama campaign that there is a price to this kind of attack politics," said the e-mail from Clinton campaign chairman Terry McAuliffe.

Obama has said previously he would fire anyone using personal attacks on his political rivals -- and the campaign quickly denounced Power's comments, even before she resigned.

"Senator Obama decries such characterizations, which have no place in this campaign," Obama spokesman Bill Burton said.

Obama blamed his Tuesday defeats in part on what he called negative campaigning. His campaign has indicated it will escalate its attacks on Clinton in response.

Click here to read the story in The Scotsman.

FOX News' Aaron Bruns and Bonney Kapp and The Associated Press contributed to this report.