Japan’s weather agency has extended its highest alert to Okinawa’s populous main island as super typhoon Neoguri approaches the southern chain, saying it may be one of the worst storms for decades.

The top-level warning on Monday means the typhoon poses a threat to life and could inflict massive damage from gusts of up to 270 kilometres per hour and torrential rain.

There are about 1.2 million residents on the main island. An earlier alert only covered the Miyako Island region with a population of 53,000.

The biggest US Air Force base in the Pacific, located on the main island, evacuated some of its aircraft as officers stressed that Neoguri may be deadly.

“People are advised to refrain from going outdoors… evacuate if necessary before violent winds occur and take appropriate action to protect themselves.”

Waves could reach as high as 14 metres, an official of the Japan Meteorological Agency said in a warning that was likely to revive memories of Japan’s quake-tsunami disaster in 2011.

The typhoon was some 500 kilometres south of the main Okinawan island at 1200 GMT (2200 AEST) and was moving north-northwest at 25 kilometres per hour.

Miyako Island, in the central area of the archipelago, was in the expected path of the massive storm.

“Record-level violent winds and high waves are posing a serious danger to the Miyako Island region,” Satoshi Ebihara, the agency’s chief weather forecaster, told an evening news conference.

“People are advised to refrain from going outdoors… evacuate if necessary before violent winds occur and take appropriate action to protect themselves,” he said.

The massive gusts and torrential rain will possibly reach mainland Japan by Wednesday, an weather agency official said earlier Monday.

The meteorological agency forecast Neoguri – whose name means “raccoon” in Korean – would dump up to 80 millimetres of rain an hour on Okinawa as it pounds the archipelago.

The storm, which could affect an area with a 500 kilometre radius, was expected to be downgraded by the time it hit the Japanese mainland.

However the Kyushu region – next to the main island of Honshu where major cities including Tokyo and Osaka are located – was already seeing heavy rain and officials warned of possible floods and landslides.

The US Kadena Air Force base in Okinawa, the largest US airbase in the Pacific, began evacuating some of its aircraft Sunday in preparation for the typhoon.