Boy refused bail over teen's sex assault

One of three teenage boys charged with the sexual assault of a 15-year-old girl in Sydney is believed to be a former child soldier from west Africa, court documents reveal.

The 17-year-old Liberian-born boy, who cannot be named, was refused bail when he faced Parramatta Children's Court on Thursday, just over two months after the girl was allegedly assaulted by three teenagers at a home in the western Sydney suburb of Mt Druitt.

Court documents obtained by AAP show a staff member at a youth care organisation believes the boy, who has no parents in Australia, "was a child soldier of war".