The disputed islands are:
The dispute results from an ambiguity over the Treaty of San Francisco. Under Article 2c),
Japan renounces all right, title and claim to the Kurile Islands, and to that portion of Sakhalin and the islands adjacent to it over which Japan acquired sovereignty as a consequence of the Treaty of Portsmouth of 5 September 1905.
The dispute is whether four of the islands currently under Russian administration were acquired by the Treaty of Portsmouth or were under Japanese control prior to this treaty. Russia had never ruled these islands before 1945.
see also Foreign relations of Japan