TOKYO (AFP) - A major fire ripped through a historic Japanese castle on the southern island of Okinawa early on Thursday morning (Oct 31), spreading throughout the World Heritage site's complex, local authorities said.

Shuri Castle is a key part of a complex dating back to the Ryukyu Kingdom, and is believed to have been in use from around the 1400s. The current structure is a reconstruction based on original plans and photos of the old castle.

"All the (three) main buildings have burnt down, with nothing left behind," Daisuke Furugen, an official with the local Naha fire department, told AFP.

"Efforts to extinguish the fire are continuing, with 30 fire engines and some 100 firefighters involved. We have no reports of injuries," he added.

The blaze started before 3am on Thursday, with the cause unknown as yet.

"The cause of the fire has not been determined yet but a security company alarm went off at around 2.30 in the morning," Mr Ryo Kochi, a spokesman for the Okinawa prefectural police, told AFP.

"It started at the main temple and looks to be spreading fast to all the main structures... Firefighters are still battling the fire," he added.

Television footage showed large orange flames engulfing the castle. Local media said there were no initial reports of injuries.

"I am extremely shocked by the initial reports of the fire at Shuri Castle," Naha mayor Mikiko Shiroma said at an emergency meeting in the regional capital broadcast on national broadcaster NHK.

"We have lost our symbol. It is a World Heritage site that represents Okinawa. More than anything, I am very worried about the fact that many Naha citizens live in neighbouring areas, and I have received reports that the fire might be threatening or affecting residents of the areas," she added.

"Naha city will make our greatest possible efforts to do everything in our power" to deal with the fire and its aftermath, the mayor said.

SYMBOL OF OKINAWA

"It's sad. It's hard to put the feeling into words," a local resident told national broadcaster NHK.

"I feel hollowed out... It's been a symbol of Okinawa."

Officials said a festival that began on the 27th was being held at the site and preparatory work for some of the event had been ongoing up until 1am, hours before the blaze erupted.

However, it was not immediately clear whether there was a link to the fire.

'A GREAT MONUMENT'

Nearly a dozen fire engines were dispatched to the scene, Kyodo news agency said, with unconfirmed reports suggesting other buildings in the complex may also have caught fire.

The castle itself was largely destroyed during World War II, but it was extensively restored and reopened as a national park in 1992.

Thanks to the faithful nature of the reconstruction, it was registered along with the surrounding complex and other Ryukyu sites in the region as a World Heritage Site in 2000.

"Five hundred years of Ryukyuan history (12th-17th century) are represented by this group of sites and monuments," the entry on the Unesco website explains.

"The ruins of the castles, on imposing elevated sites, are evidence for the social structure over much of that period, while the sacred sites provide mute testimony to the rare survival of an ancient form of religion into the modern age."

The reconstructed main hall of the Shuri castle in particular is praised as "a great monument symbolising the pride of the Ryukyu people".

Japan is dotted with historic castle complexes, most of which are careful reconstructions of original buildings.

Several have suffered damage from natural disasters in recent decades, including Kumamoto Castle in southern Japan, which was badly affected by a series of devastating earthquakes.