The disputed island was occupied by the soviet forces in the world war II particularly during Manchurian Strategic Offensive operation are under Russian admiistration but they are claied by Japan as the extension of one of the island called Hokkaido perfecture and she terms them as Northern Terriroty.
In the San Franciso treaty between the two nation, Japan should give up all the claims on island but later does not acknowledge Soviet Union as having sovereignty over the island.