Sex crimes police have appealed to a teenager to turn himself after a violent attack on a woman in Melbourne's west.
Police said a bandana-wearing male grabbed a 31-year-old woman from behind while she was walking her dog around Cherry Lake in Altona at 11am on Wednesday.
Investigators said the offender held a knife to the victim's throat and forced her into some bushes. Police believe a serious sexual assault was imminent.
A passing cyclist interrupted the attack by yelling at the offender, who fled through parklands north of the lake.
Police said the attack left the victim traumatised but she suffered only minor injuries and was not taken to hospital.
Detective acting Senior Sergeant Michael Phyland said he was concerned about the level of offending and that a knife had been used. The weapon has not been found.
"We're already investigating a serious offence and obviously if a person is armed with a knife then it could potentially get worse," he said.
"That's the reason why we're treating it so seriously and the reason we're asking for public assistance."
Police have released an image of the teen, who is believed to be frequenting the western suburbs. He is described as Caucasian, about 190 centimetres tall, with a medium build and brown wavy hair.
Police urge anyone who knows Jake's wherabouts to call triple-0.
