A US civil rights activist has resigned from the leadership of an African American advocacy group, amid a "firestorm" over allegations that she was a white person who masqueraded for years as black.

The Washington state chapter of the National Association for the Advancement of Colored People, posted Rachel Donezal's resignation letter on its Facebook page.  

In it, she writes that her commitment to the "racial and social justice movement" is undiminished but that "the dialogue has unexpectedly shifted internationally to my personal identity in the context of defining race and ethnicity." 

Ms Dolezal, 37, built a career as an activist in the black community of Spokane, Washington.

She rose to become the president of the city's branch of the NAACP and served as an independent mediator for the city's police force.

Neither position required that she be black, but media reports said Ms Dolezal identified herself in application forms as black.

She apparently resigned under pressure from the civil rights group.

"I have always deferred to the state and national NAACP leadership and offer my sincere gratitude for their unwavering support of my leadership through this unexpected firestorm," Ms Dolezal wrote.

Her parents, who are both white, said their daughter is as well, providing local media with a birth certificate and photographs of a Dolezal - who now appears tawny skinned and dark haired - as a blonde, fair-skinned child.

Ruthanne Dolezal had earlier said her daughter needed counselling to address her identity issues.

Speaking on ABC News, parents Lawrence and Ruthanne Dolezal said they were baffled by what their daughter has done.

They said they have known for years but kept quiet until reporters came to them recently and asked about their daughter.

"Rachel is trying to reject her own reality, her own identity, and by doing that she does not alter reality," Ruthanne Dolezal said.

She said that if she had the chance to speak to her daughter now she would urge her to get help.

"I would say, 'Rachel we love you. We hope you'll get the help you need to deal with your own personal issues so that you can know and believe and speak the truth,'" she said.

Her parents, who adopted four black children, said their daughter had always been interested in issues of ethnicity and diversity.

But around 2007, they learned from a newspaper article that she was claiming to be African-American.

At that point she had already severed contact with her parents.