Severe turbulence has struck a United Airlines' flight injuring several passengers and flight crew.

The pilot of United flight 1676 declared a medical emergency after sudden turbulence on approach to Billings Logan Airport, Montana.

The Boeing 737-700, which had departed from Denver with 114 passengers and five flight crew, landed without incident and was met by the airport medical team and fire department.

A United Airlines statement said three crew members and several passengers were injured, with a total of five people hospitalised.  

Passenger Bill Dahlin describing the panic aboard as the plane suddenly dropped without warning, said: "There was a lot of screaming and hollering."

It was revealed that several passengers were not wearing seat belts at the time of the incident.  

Data from the Department of Transportation reveals an average 50 passengers suffer injuries in flight every year when not wearing seat belts.