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Samantha Power, shown receiving an honorary degree at Brown University in 2007, made scathing comments about Hillary Clinton during a book tour in Europe. ((Stew Milne/Associated Press))

An adviser to Democratic presidential hopeful Barack Obama has resigned after calling rival Hillary Clinton a "monster" in an interview with a UK newspaper.

A campaign official told the Associated Press that Samantha Power's resignation is effective immediately.

The foreign policy adviser and Harvard professor made the comment in an interview with the Scotsman while on a book tour in Europe.

"We f---ed up in Ohio. In Ohio, they are obsessed and Hillary is going to town on it, because she knows Ohio's the only place they can win," the newspaper's Friday edition quoted Power as saying.

"She is a monster, too — that is off the record — she is stooping to anything," she said, trying to retract the statement in mid-sentence.

Clinton won the crucial primaries in Texas and Ohio on Tuesday, largely considered her last chance to turn around her campaign as she trailed behind Obama in the delegate count.

While Power immediately tried to withdraw her most scathing comments, she had agreed in advance that the interview with the Edinburgh-based newspaper would be on the record.

Occasionally reporters accept an exchange as "off the record," meaning it is not to be published, but that agreement must be established before the interview is conducted.

'You just look at her and think, "ergh"'

Power, a journalist herself and a Pulitzer Prize winner, issued a statement Friday in which she acknowledged the comments and said she "deeply regretted them."

"It is wrong for anyone to pursue this campaign in such negative and personal terms," she said in the statement. "I apologize to Senator Clinton and to Senator Obama, who has made very clear that these kinds of expressions should have no place in American politics."

Obama's spokesman Bill Burton wrote in an e-mail: "Senator Obama decries such characterizations, which have no place in this campaign."

In the Scotsman interview, Power went on to say: "You just look at her and think, 'ergh.' But if you are poor and she is telling you some story about how Obama is going to take your job away, maybe it will be more effective. The amount of deceit she has put forward is really unattractive."

Power became Obama's foreign policy adviser in 2005, while still continuing her job as a Time magazine columnist and Harvard professor.

She is travelling around Britain and Ireland to promote her latest book, Chasing the Flame: Sergio Vieira de Mello and the Fight to Save the World.

Power won a 2003 Pulitzer Prize for her previous book, Problem from Hell: America and the Age of Genocide.

Obama compared to Kenneth Starr

Power's comment is just one in a series of negative attacks recently exchanged by the two camps.

Obama had vowed on Wednesday to be more critical of Clinton's record and criticized her for still not releasing her income tax returns. She has said all the returns since the Clintons left the White House will be made public around April 15.

In response, Clinton spokesman Howard Wolfson said that the criticism was reminiscent of attacks led by Kenneth Starr — a U.S. lawyer reviled by many Democrats.

Starr's investigation into the Clintons' real estate dealings grew to include the Monica Lewinsky affair and his report formed the basis for Bill Clinton's impeachment.

"When Senator Obama was confronted with questions over whether he was ready to be commander in chief and steward of the economy, he chose not to address those questions, but to attack Senator Clinton … I for one do not believe that imitating Ken Starr is the way to win a Democratic primary election for president," Wolfson said.

Obama campaign spokesman Bill Burton said the comparison to Starr is "absurd."

Negative attacks blamed for Obama's key losses

Obama has blamed negative attacks from the Clinton campaign for his losses in the key Ohio and Texas contests on Tuesday.

Ahead of Tuesday, Clinton's camp seized on suggestions that an Obama aide made comments to Canadian officials that the Illinois senator's declaration he would reopen NAFTA was just political posturing. 

She blasted Obama for telling the public one thing then saying another to a foreign government in what has been dubbed "NAFTA-gate."

Prime Minister Stephen Harper's chief of staff has been accused of being the secret source who divulged to CTV journalists on Feb. 26 that the staff of both presidential candidates had quietly told Canadian officials that their statements about wanting to renegotiate the trade treaty shouldn't be taken seriously.

It's unclear why, but Obama became the main focus of CTV's story.

Soon after, Obama was again in the hot seat when someone leaked a diplomatic memo outlining a conversation in which his representative reassured a Canadian consular official that the NAFTA comments were to be seen as "more reflective of political manoeuvring than policy."

Harper has refused to heed calls to fire his top aide, but vowed to launch a wide-sweeping federal probe into the affair that will include the Prime Minister's Office.

David Wilkins, the U.S. ambassador to Canada, has said the leaked memo amounted to Canadian interference in the U.S. political process.

With files from the Associated Press