Japan was bracing Tuesday for one of its worst storms in over a decade as typhoon Neoguri barreled towards the southern Okinawa island chain, with 55,000 people urged to evacuate as the weather agency issued its highest alert.

The top-level warning means a threat to life, as well as the risk of massive damage from torrential rains and gusts of up to 250 kilometres (155 miles) per hour.

The Japan Meteorological Agency issued the alert for Okinawa's main island, home to around 1.2 million people, as well as the outlying Miyako islands.

"We advised all 55,000 people in Miyako at 10:00 pm (1300 GMT Monday) to evacuate to facilities such as community centres and municipal buildings," Miyako disaster official Katsuhiro Koja told AFP by telephone.

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

Waves could reach as high as 14 metres (45 feet), a weather agency official said in a warning that was likely to revive memories of Japan's quake-tsunami disaster in 2011.

The typhoon, which was downgraded from super typhoon status as it approached the islands Tuesday, was some 300 kilometres (190 miles) south southwest of the Kadena Air Force Base in Okinawa at 6:00 am local time (2100 GMT Monday), according to the US Navy's Joint Typhoon Warning Center.

The storm was moving north northwest at about 13 nautical miles (24 kilometres, 15 miles) per hour, it said.

"Record-level violent winds and high waves are posing a serious danger to the Miyako island region," Satoshi Ebihara, the Japanese weather agency's chief 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 rains will possibly reach mainland Japan by Wednesday, a weather agency official said Monday.

Officials urge maximum caution

The meteorological agency forecast Neoguri -- whose name means "raccoon" in Korean -- would dump up to 80 millimetres (three inches) of rain an hour on Okinawa as it pounds the archipelago.

The storm could affect an area with a 500-kilometre radius. The Kyushu region -- next to the main island of Honshu, where major cities including Tokyo and Osaka are located -- was already seeing heavy rain Monday, and officials warned of possible floods and landslides.

"I'm calling on the heads of municipalities not to hesitate in issuing evacuation warnings and don't be afraid of being overcautious," Keiji Furuya, the state minister in charge of disaster management, told a government meeting.

US officials at Kadena Air Force Base warned residents to take serious precautions.

"I can't stress enough how dangerous this typhoon may be when it hits Okinawa," Commander James Hecker of the 18th Wing stationed in Kadena said in a statement posted online.

"This is the most powerful typhoon forecast to hit the island in 15 years; we expect damaging winds to arrive by early Tuesday morning.

"So be prepared!" Hecker said. "Tie down your outdoor items and work with your neighbours to help them."

He added: "During the typhoon, do not go outside... anything not tied down, even small items, could become deadly projectiles."

Okinawa is regularly hit by typhoons but islanders were taking no chances, with Miyako fishermen bringing boats back to port and tying them down.

"It's rare that we brace for a typhoon (as early as) July," one fisherman said.