A powerful typhoon churning towards Japan's Okinawa islands has strengthened to a Class Five storm, technically the same strength as Hurricane Katrina which devastated New Orleans.

Experts says it could also threaten Japan's southernmost main island of Kyushu.

Typhoon Nabi, Korean for butterfly, increased in power to super-typhoon status, according to Tropical Storm Risk (TSR), based at London's University College.

An official at Japan's Meteorological Agency warned that the storm could reach Okinawa by Monday and curve up to hit Kyushu.

He said Nabi was expected to increase its wind speed enough to be dubbed violent, the strongest designation Japan uses for typhoons.

"It is an extremely dangerous storm, and there is the potential for quite a bit of damage," he said.

But cooler ocean temperatures near Japan mean Nabi is unlikely to have the same destructive power as Katrina, while strong prevailing winds are likely to help dissipate its force relatively rapidly once it approaches.

"The storm will be strongest when it is out over the open ocean, and by the time it nears any part of Japan it should have weakened a bit," he added.

Nabi has winds of around 180 km, near its centre, and is about 1,700 km south of Tokyo.