Five historic sites - four of them temples and shrines - across Japan were found to have been defaced with a mysterious oily liquid since last Saturday.

No arrests have been made, and police are investigating whether the cases - classified as vandalism - in Tokyo, Kyoto, Nara and Okinawa are related. The latest site vandalised was the Buddhist Zojoji Temple near Tokyo Tower.

At least a dozen stains were found there yesterday, including on a large bell, the wooden main gate and a stone figure.

This came one day after the torii gate that marks the entrance to the Meiji Shrine - a Shinto place of worship - was also found splattered.

On Sunday, a sticky oil-like substance was found on the gates of Shurijo Castle on the southern island of Okinawa. Similar stains were found last Saturday at the Shimogamo Shrine in Kyoto, a Unesco World Heritage Site, at an offertory box at the main shrine and two other buildings.

In nearby Nara, Kinpusenji Temple, another Unesco World Heritage Site dating back to the seventh century, was found to have been vandalised as well. A temple official was quoted as saying: "This is a precious building that has been inherited from older generations. It is sad to see it damaged."

The latest series of vandalism cases is similar to another cluster in 2015, when at least 24 temples and shrines across six prefectures were affected. Police then issued a warrant for a New York-based Japanese man, who claimed to be the founder of a religious body, for allegedly defacing the Katori Shrine in Chiba. However, he could not be arrested as he had left Japan by then.