A teenager who is believed to have carried out four armed robberies along a walking track in Melbourne's south-east over a three week period is being sought by police.

Casey Crime Investigation Unit detectives are appealing for public assistance to help identify an offender after a series of armed robberies. Credit:Victoria Police

Detectives believe the attacks were carried out by the same offender on the walking path at Gunns Road Reserve in Hallam between August 5 and 29.

Each incident involved the offender approaching women, threatening them with a knife and demanding their possessions.

"The teenager involved in all incidents is perceived as African in appearance, about 15-years-old with a skinny build and short dark hair," a police spokesperson said.