A desperate search for a teenager who went missing 17 days ago has intensified with the arrival of a criminal investigation unit.

Isla Bell, 19, was last seen leaving her home in Brunswick, north of Melbourne, around 6pm on October 4.

Her friends and family have since shared several photos of the young woman online and urged anyone who sees her to contact the police, particularly if she is accompanied by an “older man”.

Ms Bell’s mother, Justine Spokes, first shared her concerns online last week.

“My daughter is missing and we miss her a lot,” she wrote on Facebook.

“She leaves an impression, she’s hard to forget. She has a fighting spirit, sometimes strong and sometimes gentle, always enduring.

“We’re here for you, buddy. I love you my care bear.’

Ms Spokes later revealed the family had planned to visit the Great Barrier Reef for Ms Bell’s 20th birthday and were alarmed at not being able to contact her.

Isla Bell (pictured), 19, was last seen leaving her home in Brunswick, north of Melbourne, around 6pm on October 4.

Ms Bell (pictured) went missing 17 days ago with friends and family asking the public to help find her

‘Hey, buddy. We look forward to our trip to the reef for your upcoming birthday,” she wrote in a public message to Ms. Bell.

“We tried to call you to book flights. Let us know your thoughts.

“We love you my darling.”

A close friend of Ms Bell, Tamara Griffiths, also posted several updates on the search for the missing teenager.

She said the Fawkner Criminal Investigation Unit was assigned to the 19-year-old’s case.

“If you see anyone matching Isla’s description, especially in the company of an older man, please report them immediately,” Ms Griffiths wrote in a message.

“Do not approach or interfere – your safety is also important.

“Isla is deeply loved by her family and friends. Every share, every thoughtful look at her brings us one step closer to her safe return home.

Victoria Police are investigating Ms Bell’s disappearance and have described her as having a “thin build, red hair, a pale complexion and piercings to her nose, lips and eyebrows”.

“Police and his family are concerned for his well-being due to the length of time he has been missing,” he said in a statement.

“Isla may have traveled to the Melbourne CBD.”

Friends and family of Ms Bell (pictured) have urged anyone who sees her to contact police, particularly if she is accompanied by an “older man”.

The Fawkner Criminal Investigation Unit is believed to have been assigned to Ms Bell’s case (pictured).

Ms Griffiths shared a more detailed description of Ms Bell on Sunday, noting that she is 160cm tall and weighs 55kg.

“Long wavy red hair to the waist, multiple facial piercings and tattoos on the right leg, left thumb – symbol and left ring finger – love heart,” she said .

Anyone with information is asked to contact Brunswick Police Station on 8378 6000.