The Japan Meteorological Agency (JMA) upgraded Typhoon Hinnamnor's classification to "violent" and warned of gale-force winds and rough seas as the powerful storm approaches the island.

The typhoon had an atmospheric pressure of 920 hectopascals at its center and was packing winds of up to 270 kph as of noon on Thursday.

The agency said Hinnamnor, the 11th typhoon of the season, was expected to move slowly south of Okinawa while gaining force through Friday.

From Saturday through Sunday, the typhoon will approach the Sakishima Islands and Okinawa's main island, with wind and gust speeds increasing further, with the JMA warning of wave surges as high as 10 meters in the region.

The agency also said a weather front hitting moist air is leading to very unstable atmospheric conditions in wide regions of western and eastern Japan.

Along with thunderstorms, the agency has warned of heavy downpours and flooding in low-lying areas through Saturday.

People in the affected regions have been asked to remain vigilant for landslides, tornadoes, flooding and lightning strikes.