Seattle Police are investigating after an officer was filmed punching a girl in the face.

The uniformed officer had stopped a group of people for jaywalking - failing to use a pedestrian crossing when crossing the road - in the northwestern US city.

Ian Walsh, 39, hit the 17-year-old girl after grappling with her and a 19-year-old friend.

The clip provoked outrage after it was put on the video sharing website YouTube.

The force said the two suspects became verbally abusive and resisted arrest, but promised a thorough investigation.