By Elaine Lies
TOKYO (Reuters) - Japan was bracing on Friday for its strongest storm this year, a super typhoon powering north toward the Okinawa island chain that threatens to rake a wide swathe 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," said an official at Japan's Meteorological Agency (JMA).
"It's the strongest storm we've had this year, definitely, although it has lost some strength from its peak."
The storm, which will be Japan's second typhoon in a week, was south of Okinawa, and moving north at 15 kph (9 mph) with sustained winds of 185 kph (114 mph) as of Friday afternoon, the agency said.
The typhoon is moving extremely slowly, which raises the danger of landslides and flooding.
It was likely to be closest to Okinawa, an island chain 1,600 km (1,000 miles) southwest of Tokyo, and the home of the largest contingent of U.S. troops in Japan, late on Saturday or early on Sunday.
Television broadcast images of residents of Minami Daitojima, an island southeast of Okinawa, boarding up windows ahead of the storm.
Government officials were set to meet on Friday night to coordinate their response, Chief Cabinet Secretary Yoshihide Suga told a regular news conference.
"We are calling on all citizens to pay close attention to weather reports and respond promptly if the authorities advise them to evacuate," Suga said.
The typhoon was expected to weaken as it moved north, however, and likely to hit land on Sunday on the westernmost main island of Kyushu, before moving northeast toward Japan's largest main island of Honshu, where it is likely to weaken into a tropical storm. Tokyo was set for heavy rain, at the worst.
Tropical Storm Risk, which tracks typhoons, labeled Vongfong as a Category 4 typhoon, set to weaken to Category 2 before hitting Kyushu.
There are no nuclear plants on Okinawa, but there are two on Kyushu and one on Shikoku island, which borders Kyushu and may be hit. All are currently halted in line with national policy.
The Fukushima Daiichi nuclear plant, crippled by an earthquake and tsunami in 2011, is on the other side of the country, which is likely to see rain at the worst.
Vongfong, which means "wasp" in Cantonese, was following the path of Phanfone, a typhoon that slammed the mainland on Monday, disrupting transport and prompting evacuation advisories for hundreds of thousands of people. Seven people were killed, including three U.S. airmen swept out to sea and a man who died while surfing.
Refiner Nansei Sekiyu KK, which is wholly owned by Brazil's Petrobras, suspended marine berth operations at its 100,000 barrels-per-day Nishihara refinery in Okinawa on Thursday but crude refining operations were unaffected. More than 1,000 rescue workers stepped up a search for the last eight missing victims of the Mount Ontake volcanic eruption, hoping to make progress before the storm hits.
It was unusual for two powerful typhoons to hit Japan in such quick succession, the JMA official said, but added the overall number of such storms had not increased.
"It's more coincidence than anything else, mainly due to the way the high pressure systems are located off Japan this year."
Two to four typhoons make landfall in Japan each year.
(Additional reporting by James Topham; Editing by Clarence Fernandez)