US prosecutors have filed sexual assault charges against four boys aged nine to 14, alleging they brutally attacked an eight-year-old girl in Phoenix, Arizona, after luring her to a shed with chewing gum.

Police said the girl's parents criticised her, blaming her for bringing shame on the family. All five children are refugees from the West African nation of Liberia.

The boys held the girl down while they took turns assaulting her, police said.

"She was brutally sexually assaulted for a period of about 10 to 15 minutes," police Sergeant Andy Hill said, calling it one of the worst cases the department had investigated.

The 14-year-old boy was charged on Wednesday as an adult with two counts of sexual assault and kidnapping, the Maricopa County Attorney's Office said. He is being held without bond.