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Democratic presidential hopeful Sen. Barack Obama, D-Ill., talks with people Friday during a stop at Johnny J's Diner in Casper, Wyo.

Washington — A former adviser to Barack Obama who resigned Friday after calling rival Hillary Rodham Clinton "a monster" said Obama may not be able to withdraw all U.S. combat troops from Iraq within a year as he has promised on the campaign trail.

Samantha Power, a Pulitzer Prize-winner author and unpaid adviser, made the comments in two separate interviews with foreign media while promoting her latest book. In a tight Democratic presidential campaign where attacks are becoming increasingly bitter, Power's comments ignited a flurry of accusations between the two candidates.

Clinton said it's hard to know what Obama's real positions are, while Obama insisted he will end the war in 2009 if elected and blamed Clinton for helping start it.

The comment that led to Power's resignation came in an interview with The Scotsman. "She is a monster, too - that is off the record - she is stooping to anything," the newspaper quoted her as saying. A few hours after the comments were published, Power, a Harvard professor, announced her resignation in a statement in which she said the remarks were inexcusable and contradictory to her admiration for Clinton.

Power told RTE, Ireland's public broadcast service, that she spoke with Obama by phone Friday and he "made it absolutely clear that we just couldn't make comments like this in his campaign."

Clinton's campaign sent an e-mail to supporters telling them about the monster comment and asking for contributions to "show the Obama campaign that there is a price to this kind of attack politics."

Power's comments about Iraq came in an interview with the BBC. She said Obama's position is that withdrawing all U.S. troops within 16 months is a "best-case scenario" that he will revisit if he becomes president.

"He will, of course, not rely on some plan that he's crafted as a presidential candidate or a U.S. senator," she said. "He will rely upon a plan - an operational plan - that he pulls together in consultation with people who are on the ground to whom he doesn't have daily access now, as a result of not being the president."

Obama has actually shortened his original 16-month commitment to say he'll end the war in 2009. Obama advisers say President Bush's plan to draw troops down to 15 brigades this year means Obama could complete the removal in a year.

In Mississippi, Clinton questioned the Iraq comments based on Obama's public statements.

"He has attacked me continuously for having no hard exit date, and now we learn he doesn't have one, in fact he doesn't have a plan at all," Clinton told reporters.

Obama told voters in Casper, Wyo., that Clinton has no standing to question his resolve because she voted in 2002 to authorize the war.