They have urged Leia Cresswell to contact them to confirm that she is safe.
A Hampshire Police spokesman said: ‘Leia was last seen in Bordon at 1.15pm on Thursday July 28 and officers are becoming increasingly concerned for her welfare.
‘She is described as being white, 5ft 7ins tall, of a slim build with long, straight, dark brown hair and brown eyes.
‘She was last seen wearing a light blue hooded top and blue jeans with holes in the knees.
‘She may still be in Hampshire but we are also looking into the possibility that she might be in Surrey where we know she has friends.’
Detective Sergeant Justin Mortimore said: ‘Have you seen Leia or someone matching her description since Thursday afternoon?
‘If so, we would like to speak to you as you may have information which could help us find Leia and bring her home safely.
‘We have been following several lines of enquiry since she was reported missing but now we are turning to members of the public for their help.
‘Leia, if you are reading this, please get in touch with somebody and let them know where you are – you are not in trouble, we just want to know that you are safe.’
Anyone with information is asked to contact the Northern CID team based at Aldershot on 101, quoting 44160282105, or Crimestoppers anonymously on 0800 555 111.