West Mercia Police say Samantha Cookes, from Meole Brace, was last seen on Saturday, September 21, but was reported missing a week later, on September 28.
Police enquiries have been going on since she was reported missing and it is now thought Samantha may have made her way to Ireland.
Detective Inspector Phil Shakesheff said: "We are becoming increasingly concerned for Samantha as it is more than three weeks since she has been seen.
"Our extensive enquiries now lead us to believe Samantha has been in Ireland. More specifically we believe she has been in Dublin and also a small village called Clane in County Kildare in recent times.
Samantha is described as being around 5ft 6inches tall and as having long brown hair when she went missing.
"Anyone who believes they have seen Samantha in the last three weeks, or who may have information about her whereabouts, is asked to call West Mercia Police as soon as possible on 101 or 0300 333 3000."