The heads and bodies of at least five people have been found in a wooden boat thought to have come from North Korea.
The gruesome discovery was made after the small vessel washed up close to the Japanese island of Sado, authorities in the country said.
Two decapitated heads and five bodies were found in the stern of the boat, coastguard official Kei Chinen told news agencies.
But such was the scene that met police, he added, it was not immediately possible to confirm whether the heads belonged to the bodies or were from two other people altogether.
The discovery came after an officer first spotted the wooden boat – which featured Korean numbers and letters – on Friday afternoon.
Officers waited until Saturday before boarding it due to unstable weather.
Investigators are now working to identify the cause of death and understand where the corpses might have come from.
It comes amid heightened tension between North Korea and Japan.
Kim Jong-un’s promise of a “Christmas gift” for US president Donald Trump – widely interpreted to mean he was planning new ballistic missile tests – were met with dismay in neighbouring Japan earlier this month.
On Friday, the Tokyo-based public broadcaster NHK posted a news bulletin that incorrectly reported North Korea had launched a missile that fell into waters east of the Japanese archipelago.
The station later issued an apology saying it was a media training alert.
