A manhunt is underway after a 12-year-old girl was slashed in the face by a man armed with a knife in a motorway underpass south of Brisbane on Thursday morning.

The girl was attacked about 7.40am while walking along a pedestrian walkway in the Logan suburb of Kingston.  Police did not believe the victim knew her attacker.

The girl suffered a large cut to the side of her face and was taken to the Queensland Children's Hospital in a serious but stable condition, according to police.

Her attacker was described as Caucasian in appearance, about 17 or 18 years old, with a proportionate build, narrow face and short, light brown hair. He was wearing a fluorescent yellow  collared shirt and pants or jeans.

Detectives from the Logan Child Protection and Investigation Unit are investigating.