The number of dead birds covered in oil has risen to 4,000 on the shores of Hokkaido and the cause remains a mystery, prefectural officials said Friday.
Officials of the town of Shari in the Abashiri district said an additional 1,600 birds were found dead in oil on the shores of the Shiretoko Peninsula on Thursday as snow melted in the area.
The officials said more could be found in snow and ice in the area.
Dead birds covered in oil were first found in Abashiri in late December, and more cases followed in coastal areas of the peninsula, which is on the list of the World Heritage Sites of the U.N. Educational, Scientific and Cultural Organization.
Local officials have said the birds were mostly crows and sparrows.
The prefectural government has said about 10 cu. meters of kerosene leaked into the sea after an oil spill accident that occurred in mid-December on Sakhalin.
Some experts say the oil slick could have made its way to the northeastern Hokkaido coast along with drift ice.
Others are skeptical, saying such a large number of birds would not have died due to the amount of oil that leaked from the accident.