Two teenagers have been accused of sexually assaulting a vulnerable boy with a toilet brush handle, burning him with cigarettes, and suffocating him, in a 36-hour ordeal.

A 15-year-old boy has claimed that he was sexually abused in a toilet at Bristol Children's Hospital by 18-year-old Michaela Murch and another teenager, who cannot be named due to legal reasons.

Murch and her alleged accomplice have denied the assault, on April 11 this year, and are standing trial at Bristol Crown Court.

They have both admitted charges of putting a person in fear of violence, two counts of assault occasioning actual bodily harm and two counts of assault by beating.

It is alleged that the boy was sexually assaulted at the hospital after receiving treatment for burns inflicted by the two defendants the previous day.

The boy said the pair tried to suffocate him with a plastic bag before attacking him with a toilet brush, and Murch's co-defendant took to the stand to accuse Murch of the sexual assault.

Murch declined to give evidence while her co-defendant told the court that, having broken the toilet brush used it to assault the vulnerable boy who was bent over the toilet, reported the Bristol Post.

Murch, of Silbury Road, Ashton, allegedly began the abuse the day before after the boy accompanied the two defendants to visit a relative at St Michael's Hospital, in Bristol.

At some point after the visit the boy claims he was forced to remove his clothes and run in the street naked, which was captured on CCTV.

The court heard that the pair burnt the boy's leg using a lighter and put cigarettes out on his feet, leaving him needing hospital treatment.

The Post reported that the court heard how the boy also had a plastic bag placed over his head in an attempt to suffocate him, the pair only ceasing the horrific abuse when he started sobbing.

Murch did not take to the stand, but her co-defendant said through her defence lawyer William Rose they were "sorry" for the charges they had earlier admitted to.

The trial continues.