A 14-year-old boy was approached by a man wearing a balaclava in Locks Road, at 7.35am on Thursday February 6.
As the teenager ran away, he turned around and described seeing the male holding a knife.
A spokesman from Hampshire Constabulary said: ‘The male is described as being 5ft 10ins tall, white, of a slim build, and he was wearing a thin black coat.
‘He was also wearing black gloves and a black balaclava.
‘Officers have been carrying out house-to-house enquiries in the area since the incident, as well as analysing CCTV.
‘They are keen to speak to anyone who may have been in the area and saw someone acting suspiciously.
‘You may have been driving through the area at the time of the incident and either saw something or captured something on your dash cam.’
The police are asking for anyone who witnessed anyone acting suspiciously in the area at the time to call 101, quoting 44200046172.