Japan was bracing on Friday for its strongest storm this year, a supertyphoon powering north toward the Okinawa island chain that threatens to rake a wide swath of the nation with strong winds and torrential rain.

Typhoon Vongfong, which at one point rivaled last year's devastating Haiyan in strength, was weakening slightly as it moved across the open ocean, but still packed winds gusting as high as 259 kph (160 mph).

"There is no question that it is an extremely large, extremely powerful typhoon," an official at the Meteorological Agency said of the 19th typhoon of the year.