A major fire ripped through a historic Japanese castle on the southern island of Okinawa early on Thursday morning, 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.

The blaze started before 3:00 am on Thursday morning, 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,” Ryo Kochi, a spokesman with 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.

“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.

Kochi said a tourist event was being held at the castle from the 27th, and some work linked to the event continued until 1:00 am but it is not clear whether that was linked to the fire.

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.