The family of Amari Allen apologized for the pain and anxiety caused by her false claims. (AFP)

A 12-year-old black girl in the United States has confessed she falsely claimed three white boys pinned her down and cut off her dreadlocks in a supposed attack that sparked outrage.

Amari Allen, a pupil at a private school in Springfield, Virginia, said she was assaulted in break time by fellow pupils who covered her mouth, called her ugly and cut off some of her hair.

But her family and the Immanuel Christian School issued statements on Monday saying she had admitted that the story was untrue.

“To those young boys and their parents, we sincerely apologize for the pain and anxiety,” the family said.

“We hope and pray that the boys, their families, the school and the broader community will be able to forgive us in time.”

School principal Stephen Danish said the student acknowledged that the allegations were false and he thanked the police for investigating.

“This ordeal has revealed that we as a school family are not immune from the effects of deep racial wounds in our society,” he added.

Security video footage disproved the allegations, but the girl maintained she had been a victim of bullying at the school, local media said.

She admitted she had cut her own hair and made up the assault after her grandmother, who is her legal guardian, questioned her last week.

The girl and her family had given interviews to TV channels and newspapers.

The traditional Christian school, 15 miles (24 kilometers) outside Washington, was in the news earlier this year when Karen Pence — wife of Vice President Mike Pence — took a part-time art teaching job.

US television actor Jussie Smollett has been at the center of an alleged fake assault after claiming he was attacked in the middle of the night in January.

The black, openly gay actor has denied accusations that he hired two acquaintances to stage a racist and homophobic attack to gain publicity.