US Democratic front-runners Hillary Clinton and Barack Obama have traded accusations over a photo of Mr Obama circulating on the internet.
The photo of Barack Obama was taken during a 2006 trip
The picture, sent to the Drudge Report website, shows Mr Obama wearing traditional Somali dress during a visit to Kenya in 2006.
The website said it was circulated by Mrs Clinton's staff but her team denied they had sanctioned its release.
The row comes as the rivals campaign for two crucial primaries next week.
Analysts say Mrs Clinton needs to win the contests, in Texas and Ohio, to remain in the race to choose the Democratic candidate for November's presidential election.
With tempers fraying ahead of a crucial debate in Ohio on Tuesday, the former first lady highlighted Mr Obama's lack of foreign policy experience during a speech in Washington.
Reminding the George Washington University audience of her own international credentials, Mrs Clinton suggested her rival would need a "foreign policy instruction manual" to keep the country safe if elected.
The photograph published on Monday shows Mr Obama - whose father came from Kenya - wearing a white turban and a white robe presented to him by elders in the north-east of the country.
The Drudge Report said the image had been circulated by "Clinton staffers" as a smear.
Addressing the issue briefly in an interview with a Texas radio station, Mr Obama said: "I think the American people are saddened when they see these kind of politics."
Some Clinton aides have tried in the past to suggest to Democrats that the Illinois senator's background might be off-putting to mainstream voters.
A campaign volunteer was sacked last year after circulating an email suggesting, falsely, that Mr Obama was a Muslim.
But the BBC's Justin Webb in Ohio says the photograph - coming at this pivotal moment in the campaign - is being seen by the Obama team as particularly offensive.
Mrs Clinton has stepped up her rhetoric in recent days
His campaign manager, David Plouffe, accused Mrs Clinton's aides of "the most shameful, offensive fear-mongering we've seen from either party in this election".
The accusation was dismissed by Mrs Clinton's campaign manager Maggie Williams.
"If Barack Obama's campaign wants to suggest that a photo of him wearing traditional Somali clothing is divisive, they should be ashamed," she said.
"Hillary Clinton has worn the traditional clothing of countries she has visited and had those photos published widely."
In Monday's speech, the New York senator characterised Mr Obama as a rash and inconsistent politician with the same sort of inexperience that President George W Bush had when first elected.
The US had already suffered the "tragic result" of electing a president ill-versed in geopolitical affairs, she told supporters in Washington DC.
Meanwhile, the New York senator's communications chief Howard Wolfson was also on the offensive, speaking out against what he called Mr Obama's "entirely negative" campaign.
"He has run against her as the status quo, he has essentially called her divisive, he has called her untruthful, he has questioned her credibility," said Mr Wolfson.
"He has said she will do or say anything to get elected. Now, if that's not negative, I don't know what negative is."
Commentators suggest Mrs Clinton needs strong victories in both Ohio and Texas to keep her White House campaign alive.
But several polls suggest she is trailing Mr Obama.
The Illinois senator is leading in Texas for the first time, according to a CNN poll, with 50%, compared to 46% for Mrs Clinton.
Mr Obama has won 11 consecutive primaries and caucuses in recent weeks, and is now seen as the Democratic front-runner.