Russian President Vladimir Putin visited Iturup Island in the disputed Kuril Islands, prompting strong protests from Japan.
The Kuril Islands are a volcanic archipelago of 56 islands, stretching about 1,200 km in the North Pacific.
Japan claims four islands—Iturup, Kunashir, Shikotan and Habomai, known as the Northern Territories, while Russia administers them as part of the Sakhalin region.
Russia took control of the islands after World War II, while Japan continues to claim sovereignty over the four islands.
Russia cites post-war agreements such as the Yalta Agreement and Potsdam Declaration (1945), while Japan refers to earlier treaties including the Treaty of Shimoda (1855) and Treaty of St. Petersburg (1875).
The dispute has prevented the signing of a formal Russia-Japan peace treaty since World War II.