Several passengers and crew members were injured when a Cathay Pacific jumbo jet hit turbulence over northern Japan, the airline said Wednesday, as one passenger likened the experience to a "rollercoaster".

The Boeing 747-400, carrying 321 passengers and 21 crew, was rocked by sudden turbulence when it flew over Hokkaido prefecture Tuesday.

Cathay Pacific said two crew members and a number of passengers were injured, but did not confirm a total of 12 injuries reported by the South China Morning Post newspaper.

The flight from San Francisco to Hong Kong landed at the southern Chinese city's airport at 6.26 pm local time to be greeted by a fleet of ambulances and emergency vehicles.

One of the injured was taken to hospital on a stretcher.

The turbulence was "more intense than riding a rollercoaster", a passenger was quoted by the newspaper as saying.

The plane "was jolted for at least two minutes. I was thrown up very high," he said, adding the turbulence started very suddenly before any warning.

The airline said medical assistance was provided to the injured on board before the flight landed.

"Hong Kong Civil Aviation Department has been informed of the incident, and we are collaborating with the authority on the investigation," a spokeswoman said.

Cathay Pacific is Hong Kong's flagship airline.