Japan's largest ancient mound, Daisen Kofun, officially designated as the tomb of Emperor Nintoku, was at least about 40 meters larger when it was first constructed in the fifth century, Imperial Household Agency officials said Thursday.

The agency's survey of the water-filled moats surrounding the massive tomb in Sakai, Osaka Prefecture, has revealed the mound was at least 525 meters in length, up from the current official measurement of 486 m.

The tomb is already known as one of the three largest in the world, together with the Mausoleum of the First Qin Emperor in China and the Great Pyramid of Khufu in Egypt.

The mound's triple moats are believed to have been empty or carried a lower level of water at the time it was built. An agency official said a base even larger may be buried under the sludge accumulated over some 1,600 years.

The discovery comes as Japan seeks to register the Mozu-Furuichi ancient tumulus cluster covering the site as a UNESCO World Cultural Heritage site this summer.

"Regardless of World Heritage registration, the tomb of Emperor Nintoku is very important as an emperor's burial site and as a cultural asset," said an agency official in charge of the survey, adding the agency will conduct surveys needed for protecting the site.

The agency bans people from entering the site and does not hold excavations and other surveys in principle.

The latest survey using sonars and lasers was conducted in December 2016 as the agency hopes to start bank protection work to stop erosion of the mound and needed information on the amount of water in the moats and the landscape underneath the water.