Zeituni Onyango has been living in public housing in Boston, AP says
Democratic presidential candidate Barack Obama has said he was not aware that he had a Kenyan aunt living illegally in the United States.
The Associated Press says Zeituni Onyango, half-sister of Mr Obama's late father, lost a bid for asylum in 2004.
Ms Onyango lives in public housing in Boston, the news agency says.
The Obama campaign issued a statement: "Senator Obama has no knowledge of her status but obviously believes that any and all appropriate laws be followed."
Ms Onyango, referred to as "Auntie Zeituni" in Mr Obama's memoir, was instructed to leave America by a US immigration judge who had denied her asylum request, a "person familiar with the matter" told AP.
The source spoke on condition of anonymity because "no-one was authorised to discuss Onyango's case".
According to AP, Ms Onyango has not been mentioned in the presidential candidate's speeches and "is not someone who has been part of the public discussion about his personal life".
David Axelrod, Mr Obama's chief media strategist, told reporters he believed that people would be "suspicious about stories that surface in the last 72 hours of a national campaign".
Mr Obama's father grew up in Kenya herding goats but gained a scholarship to study in Hawaii where he met and married Mr Obama's mother, who was living in Honolulu with her parents.
The house where the aunt lives
Barack Obama Senior left his son when he was two years old and lived most of his life in Kenya, where he fathered six other sons and a daughter with three other wives. He died in a car crash in 1982.
The presidential hopeful first met his father's side of the family when he travelled to Africa 20 years ago.
Describing the visit in his memoir, he talks of "Auntie Zeituni" being "a proud woman".
Mr Obama's campaign was quoted by AP as saying he had seen her a few times since that meeting, beginning with a return trip to Kenya with his future wife, Michelle, in 1992.
Ms Onyango visited the family in Chicago on a tourist visa at Mr Obama's invitation about nine years ago, the campaign added, stopping to visit friends on the East Coast before returning to Kenya.
She attended Mr Obama's swearing-in to the US Senate in 2004 but campaign officials say the politician provided no assistance in getting her a tourist visa and does not know the details of her stay.
The campaign added that Mr Obama had last heard from her about two years ago when she called saying she was in Boston, but he did not see her there.
The Obama campaign also said it was returning $260 that Ms Onyango had donated in small increments to her nephew's presidential bid.
Under federal election law, only American citizens and certain Green Card holders are allowed to make donations to campaigns.
Residing in the US in violation of immigration law is not a criminal offence and such cases are therefore handled outside the criminal court system.
Ms Onyango listed her employer as the Boston Housing Authority and last gave $5 on 19 September, AP says.
Many experts estimate there are more than 10 million illegal immigrants in the US.