A fire has destroyed the main structures of Shuri Castle, a Unesco World Heritage site on Japan's southern island of Okinawa.

Firefighters have been battling the flames since early on Thursday. No injuries have been reported so far.

The wooden castle, built 500 years ago in the Ryukyu Dynasty, was designated Japan's national treasure in 1933.

It was almost completely destroyed in a battle during World War Two. The current structure is a reconstruction.

The main building, as well as the north and south structures of the castle have burned to the ground, local media said.

"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 news agency AFP.

Shuri Castle sits on top of a hill overlooking the city of Naha, Okinawa's capital, and is surrounded by curved stone walls.

It served as a campus for Okinawa's largest public university until the 1970s, and has been a popular tourist attraction since.