The storm could be one of the worst in decades, with gusts of up to 270 kilometres per hour expected to slam into the southernmost subtropical island chain early on Tuesday, and possibly reaching mainland Japan by Wednesday, the national weather agency said.

The typhoon was located some 600 kilometres south of Okinawa's main island at 0300 GMT and was moving north northwest at 25 kilometres per hour.

"Please be vigilant, stay inside buildings and avoid working outside and making unnecessary trips," a meteorological agency official said at a press briefing.

The official warned that the storm could pack "record level" winds and stir up high waves.

"It is likely to arrive in Okinawa on Tuesday morning, generating violent gales and high waves... and, in some areas, violent rains," he said.

"Please be ready to evacuate," the official added.

The meteorological agency forecast Neoguri, whose name means racoon 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, Kyushu region -- next to the main island of Honshu where major cities including Tokyo and Osaka are located -- was already witnessing heavy rains and officials warned over 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.

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

"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!...Tie down your outdoor items and work with your neighbours to help them."

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

(Agencies)

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