The girls were walking home when the vehicle pulled up beside them in Pelham Street at about 3.45pm.
The man reportedly asked the pair to get inside the van but the girls ran off and alerted their parents. Neither girl was physically injured.
The man is described as being white, in his 50s with grey hair.
Officers would also like to speak to anyone with knowledge of a man matching the description that owns red van in the Manton area or anyone who may have been in the area at the time.
Witnesses or anyone with information is asked to call County North CID ON 0300 300 9999 or Crimestoppers anonymously on 0800 555 111.