Russia has denied entry to disputed islands to a mayor from Hokkaido in retaliation for sanctions imposed by Japan over the Ukraine crisis, diplomatic sources said on Saturday. Nemuro Mayor Shunsuke Hasegawa was disallowed to join a five-day study tour through July 1 to assess the potential for joint economic activities on the islands off the northernmost prefecture that are controlled by Moscow but claimed by Tokyo, the sources said. The Russian government introduced the retaliatory measure in August 2014, but the number of Japanese people subject to it and their names were not released at that time. In April that year, Tokyo had announced it would deny visas to 23 Russian nationals as part of the sanctions over Moscow’s annexation of the Crimea region in Ukraine. Hasegawa is believed to have been included on the blacklist as he plays a key role in Japan’s movement to seek the return of the islands called the Southern Kurils in Russia and the Northern Territories in Japan. Can Russia and Japan resolve the Kurils territorial dispute? The isles of Etorofu, Kunashiri, Shikotan and the Habomai islet group were seized by the Soviet Union after Japan surrendered in August 1945, ending the second world war. As the joint feasibility study on the islands was a visa-free arrangement, the Japanese government claimed Hasegawa should be allowed to join the tour but Russia did not concede, according to the sources. The Japanese Foreign Ministry has not disclosed any reason why Hasegawa was excluded from the tour. The mayor has expressed disappointment over the matter. Japan hopes joint economic activities, once started, will pave the way for addressing the decades-long territorial dispute with Russia.