Truck driver hit by 'projectile' in Sydney

A truck driver has been injured after being hit by an unknown object while driving in Sydney's southwest.

The man was sitting in his rig at traffic lights on Heathcote Road at Wattle Grove on Wednesday evening when police say he was struck by a "projectile".

He suffered a minor wound to his head and was taken to Liverpool Hospital for treatment.

Officers do not know what injured the man and are appealing for witnesses.