Some Clinton aides have tried in the past to suggest to Democrats that Barak Obama'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 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.

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. [...] Mrs Williams did not address the question of whether staffers circulated the photo. [...]

The photograph 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.

—Robed Obama picture ignites row