A US-bound American Airlines flight was forced to make an emergency landing in Tokyo after hitting severe turbulence, leaving several people injured.

Passengers said the plane dropped suddenly and rocked for half a minute.

"Some people were walking around and might have hit their heads because it was a pretty big drop."

The turbulence was caused by a winter storm in northern Japan that's brought heavy snow and winds gusting at typhoon speeds.

Japanese media said more than 300 flights have been cancelled across the country.