A13-YEAR-OLD girl was left traumatised after an older teenager tried to sexually assault her.

A group of three boys aged 16 to 18 forced the girl into a Brighton alleyway at about 8pm on July 31, before one of them attempted to touch her under her clothes.

The boys then ran away, leaving the girl very upset but not physically hurt.

She had been walking north along Portfield Avenue, Hollingbury, when she was attacked.

The boys were all described as white and aged between 16 and 18.

One was about 6ft tall. He had short brown spikey hair, and wore a black t-shirt, dark blue shorts, possibly denim, cut off above the knee, white soled trainers, black on the outside with a red tongue and black laces.

Another was about 5ft 10in tall, with black or brown hair, slightly tanned, of slightly bigger build, wearing a black or very dark blue T-shirt – possibly Polo brand - with two white buttons at the top, a shirt with thick material, short sleeved with no collar, denim shorts, and black trainers with white laces.

The third boy was about 6ft tall, with blond or brown tousled hair, wearing a short sleeve black/dark blue top, black Nike shorts with a white tick on the leg, a blue watch, either a FitBit or an Apple.

Detectives are seeking a local woman they believe may have valuable information.

Detective constable Leianne Blampied said: “If you were in that area at the time and saw anything suspicious we would like to talk to you.

“In particular we would like to trace a woman believed to be in her fifties or early sixties, short and slim with greying hair and wearing a floral dress and a jacket. She walked past as the boys were holding the girl in Portfield Avenue. She stopped and spoke to the boys.

Call police on 101.